Hi,

In my opinion, several things may catch someone's attention although it's never black or white. Cautious must be observed when analysing this kind of topic. According to my small experience, I would be attentive to following points :

• Change of address of the publisher or registered office ;
• Lots of spams received suddenly ;
• Community of researchers noticing the changement ;
• Warnings from the Beal List/DOAJ... ;
• Open Acess conditions have been modified.

Note that I wrote this according to my experience and not from rigorous researches on the topic.

Best regards,