how to know if an agency is real and not a scam?

Hi! I just applied for a chatting role, and I am now in the training phase. The agency provided materials for us to learn with, and shadowing also starts next week. My concern is: Is this too good to be true? The manager that I talked to seems legit, he even really ran through my profile from OFMJobs(dot)com and saw I'm a web developer. He is also active on social media, and has a LinkedIn account. They are transparent with the paying schedule and the rates but IDK, just seems too good to be true. What red flags should I also look out for? Thanks!