Borrowing from researcher César Jiménez-Yanez:

Regularly predatory journals have the following characteristics:

Send emails regularly inviting to publish

They offer impact factor and show medals and high indicators

Offer to publish in a very short time (days or weeks)

They are multidisciplinary, that is, they accept jobs from all areas and disciplines.

Charge to be published (high costs)

They do not detail the arbitration process (sometimes they do not even mention it)

Present the DOI or Crossref logo as something "important"

The name of the magazine regularly looks like a serious academic journal

They are not affiliated with COPE (https://publicationethics.org/)

You can check the ISSN of the magazine in the MIAR database http://miar.ub.edu/

You can check the ISSN of the magazine in the SCIMAGO database https://www.scimagojr.com/

You can check the ISSN of the magazine in the Clarivate Analytics database http://mjl.clarivate.com/

Recently have been contacted about 15 times by a supposed journal with a) a generic name and proposing to re-publish every time a new paper offering a higher impact. They start saying "congratulations for your great work" or something alike. b) why to republish, if that papers are already available (e.g. through this portal) c) I am asked to pay around US$ 90 for that service.

Impact indexes are not necessarily a good idea as bad research can simply become popular through proper marketing. They are generating academic noise, as well as several predatory conferences do it.

Borrowing from researcher César Jiménez-Yanez:

Regularly predatory journals have the following characteristics:

Send emails regularly inviting to publish

They offer impact factor and show medals and high indicators

Offer to publish in a very short time (days or weeks)

They are multidisciplinary, that is, they accept jobs from all areas and disciplines.

Charge to be published (high costs)

They do not detail the arbitration process (sometimes they do not even mention it)

Present the DOI or Crossref logo as something "important"

The name of the magazine regularly looks like a serious academic journal

They are not affiliated with COPE (https://publicationethics.org/)

You can check the ISSN of the magazine in the MIAR database http://miar.ub.edu/

You can check the ISSN of the magazine in the SCIMAGO database https://www.scimagojr.com/

You can check the ISSN of the magazine in the Clarivate Analytics database http://mjl.clarivate.com/

Yes, you can get around predatory journals. We looked into the traps and obstacles for you and came up with a set of 25 criteria that can help you to identify unethical or deceptive behaviour in journal publications. 

The list below is a selection of what we think are the most prominent and easy to spot indicators. We looked through the various available lists of criteria to identify predatory journals: One provided by Beall, Eriksson & Helgesson (2017), Rele et al. (2017), Shamseer et al. (2017), the ‘Think, Check, Submit’ initiative and the anonymous initiative Stop Predatory Journals. From the multitude of criteria (over 65 items) provided, we made a concise selection for you that should enable you to check yourself. 

1. Selected criteria of predatory journals 

#1: The journal asks for a submission rather than publication fee. Even if your paper does not get accepted, you will still have to pay the fee. The fees are typically rather low compared to established, legitimate journals (e.g. < EUR 150). 

#2: The journal promises a very fast publication process. Typically 72h to a very few weeks, which makes it rather unrealistic that a proper peer-review process is going to be applied. 

#3: The journal defines no clear publication date. There is no regular publication activity visible. The journal does not state a fixed number of issues and volumes to be published per year. 

#4: The journal claims or seems to be rather new. It has a very low number of published papers and issues so far. 

#5: The aims and scope of the journal are not defined or rather vague. 

#6: The journal publishes papers that are far outside the scope of the journal. 

#7: The journal requests manuscripts to be submitted via email.

#8: The journal has a very high acceptance rate of papers. Papers are also accepted within very short time. 

#9: The paper titles and abstracts contain errors. 

#10: The journal claims to be very international but has no or very few papers from well-known or international authors. 

#11: The journal has a very small editorial board or an editorial board that is still to be announced. The names mentioned are not well-known researchers. 

#12: The journal does not list contact addresses of editors, editorial board members or the publisher. Contact addresses can also be incomplete. 

#13: The Editor-in-Chief of this journal is also the Editor-in-Chief of (many) other journals with widely different subjects.  

#14: The journal has a poor and unprofessional looking website. It is not very informative and contains factual mistakes and language errors. 

#15: The journal sends open calls/unsolicited emails to many authors to invite them to submit papers to them. 

#16: Communication from the journal includes poor language, spelling mistakes or grammar errors. 

#17: The journal is not transparent about the editorial policies that they apply. 

#18: The journal claims to have an impact factor despite the fact it is a new journal in which case it cannot have an impact factor yet. 

#19: The journal requires authors to transfer copyright despite claiming it is open access. 

#20: The publisher of the journal is not a member of a recognised professional body that commits to best practices in publishing, e.g. 

• Committee on Publication Ethics (COPE)
• International Association of Scientific, Technical, & Medical Publishers (STM)
• Open Access Scholarly Publishers Association (OASPA)
• European Association of Science Editors (EASE).

#21: The journal is not indexed in any of the established journal databases such as 

• PubMedCentral
• Scopus or 
• Web of Science. 

#22: The journal’s contact email address is non-professional and non-journal affiliated (e.g. @gmail.com or @yahoo.com). 

#23: The journal is listed on Beall’s list of predatory journals or any other journal blacklist. 

#24: The journal (if it is open access) is not listed on the Directory of Open Access Journals (DOAJ). The DOAJ is a kind of whitelist for open access journals as certain criteria must be met in order to be listed.  

#25: The journal is not read or known by your colleagues. They do not publish in nor read this journal. 

There have been similar discussions before with interesting contributions:

https://www.researchgate.net/post/What_are_predatory_journals_How_to_find_them_Are_the_articles_published_in_them_considered

https://www.researchgate.net/post/How_to_spot_the_predatory_journal

https://www.researchgate.net/post/How_to_know_about_predatory_journals

See also https://beallslist.net/ , mentioned in a new discussion: https://www.researchgate.net/post/What_is_the_accuracy_level_of_Bealls_efforts_https_beallslistnet_in_tracking_predatory_journals_across_the_world

1) In case of predatory journals...they don't share their NAAS rating...second they will publish your paper without sending it to the reviewer...and get your paper published in their journal as such without and ifs and buts

Thanks for the information Aloísio Leoni Schmid

Anton Vrdoljak Thanks for detailed information.

Abdullah Al-Oshoush Thanks for the detailed Criteria.

Wolfgang R. Dick Thanks for the useful link.

Rifat Bhat

Thanks for the useful information.

Alaa Ali

dear Pankaj Sharma

You can check the predatory list:

www.beallslist.net

another way: if you get acceptance without any comments just asking you to pay the APC, this means the journal is predatory.