They can be disguised as indy or agency, so none of that matters when choosing. If you are seeing girls listed as under 21, I would ask for an ID...know plenty of guys who have done this. She can cover her damn name, but let you see the year she was born. Sadly, human trafficking is not just a myth and it exists both in and outside of the U.S. It is a multi-billion dollar industry, so your best bet is to try and weed out younger girls who have the possibility of being pimped or forced.
The short answer to your question is no, you can't tell unless there are some serious red flags. Even then, she may just have a pimp. Reading reviews and having VIP do nothing to protect you from seeing a girl who is underage or forced unless the guy are actually allowed to say so in the review lol. Usually, these reviews are pulled because it puts the board at risk having such reviews. Go with your gut.