Teshome - essentially, 'red flag' number 1 with predatory journals is always be very wary when any journal approaches you directly. There are some established open access journal corporations i.e. SAGE, BMC Central - but most people know that they are established and they mostly send out generic invitations -more based on 'do you know this journal/suite of journals exists - related to your topic area?'. If you are not sure - then Directory of Open Access Journals (DOAJ) is a good site to check first. If the journal targeting you is not on there - it's another red flag. Then there are many other red flags - such as:
Poor quality online interface.
Minimal and/or very broad journal scope i.e. we publish almost anything.
Poor English quality and grammatical errors.
Check the country of origin.
Unknown editors.
Unknown or no editorial board and/or all based in the country of origin
Unsophisticated online manuscript submission processes i.e. send a word document by email
Upfront publishing charges
Not registered or associated with any reputable professional bodies and/or citation agencies.
Check the quality of existing articles in the journal.