Unfortunately there are no reliable listings of scam (pseudo-)"agencies". Even for example BBB (Better Business Bureau) listings or official licenses legally required in some locations like for example France are not an absolute guarantee for fairness. Further some not-so-legitimate "businesses" appear overnight and mysteriously vanish later. Often the same persons operate several businesses and relocate from time to time.
Best is to develop a good feeling to detect scams as there's often a
whole set of more or less obvious details (red flags) which will allow you to determine if an agency is legit or not. Some common sense will already help a lot, just make sure to not be blinded by some fairly well trained people who'll try to make you feel very special.
It's not always easy to define what is a scam or not. If you pay for overpriced useless low-quality tests or to be listed online by some random online-only pseudo agency it is not necessarily formally illegal if there are no false promises, you just paid too much for something useless (but about the same happens most of the time when you purchase a lottery ticket).
Another point concerns names. Some major agencies have offices in various cities but not all. You can find some agencies in small to zero market areas which have the same (or a very similar) name as some known agencies but which are totally unrelated to the legit agency (not to be confused with licensing which can be some sort of perfectly legit franchising system, there are some odd license practices when it comes to some modeling schools though).
If you ever get contacted online, be it by e-mail or Private Message, make absolutely sure that the sender is legit. In case of doubt find the agency phone number or e-mail mentioned on their website (not some other online source) and just shortly call or e-mail them.
If an e-mail address is directly something like @agencyname.com (but make sure the domain name after the @ is 100% identical with the agency website, pay attention to dashes and what follows the dot) it's likely to be legit.
Free e-mail addresses like e.g. @hotmail.com etc. are not necessarily scams though often they are. Some legit scouts, bookers and even a few legit agency offices use such e-mail addresses for various practical reasons.
Overall most scams can be avoided with some common sense but unfortunately it's not always that easy. There are a couple of known schemes but new tricks appear from time to time.
Follow you instinct, if something seems fishy it probably is and even it if is not, if you've a bad feeling, it's likely that it won't work well. Modeling is a lot about personal relationships, trust, confidence and communication so if you don't "feel" something you should be careful.
Blunier
(I'm not a native English speaker so I apologize once for my English as this is my first tFS message.)