And for writers getting approached by what may seem like professionals:
Writers Beware is an archive of scammy editors, agents, publishers, contests, etc. Has a lot of great resources.
And as a general advice: If someone approaches you, google them. Chances are, someone out there has written about them if they're scammy (Something like Dreame/Ficfun for example are notorious for poaching unknowing authors and getting them stuck in rights-grabby contracts and there's been written extensively about them online)
There's usually also reviews of apps - so if you're approached by one, check out the reviews. The bad ones often have very bad reviews since it's all about capitalising on people who might not know better.