A lot of the stuff that gets categorized as "trafficking" is local pimps using underage girls, and a significant number of them are doing it voluntarily.
In Canada, trafficking includes cross province movement. Underaged girls are travelling from Nova Scotia to Ontario to work, often with a pimp (or a manager, or whatever). People imagine trafficking to be eastern european women coming in a stream out of a van into a seedy location, and being beaten and coerced into working. That's just a more titillating story than runaway teenagers travelling to another province, working in isolation, in an industry they may choose but do not understand and are not equipped to work safely or smartly in because it's shrouded in so much myth and secrecy. The salacious myths and imagined misery make it hard to focus energies on the individuals who really do need help.
As others have suggested, using common sense will go a long way toward preventing you from using someone who's been trafficked. For example, if she looks too young, err on the side of caution. If she seems like she doesn't want to be there, she may be being forced to do the work. If she's got bruises on her, she may have been beaten and is being forced to work. And so forth.
What do you do in this situation?
I was talking to a woman who had a pimp (not trafficked, not physically abusive, not working against her will) who put so much emotional pressure on her when she didn't make more money than the negotiated price (what could you have done to make more money? why didn't you get a bigger tip?)... she never told me whether or not she ever had a client who didn't pay her, or didn't go through with the date, but I don't think her pimp would have been pleased. Even though there was no physical abuse, she described the pressure and the stress as being traumatic.
She did mention that she was not aware of places she could have gone for help. She was aware of outreach groups for survival sex workers, but she was not a survival sex worker, so she didn't feel those groups were for her. When she was working, she didn't want to get out of the industry (she did leave eventually), but she did tell me that if she had known of outreach groups that were equipped to educate women in how to work safely (rather than how to exit when they didn't want to or weren't ready to), things might have been different for her.
If I were a client and concerned about coming across women in these situations, I would make sure I was equipped to help them. Where I live, the name of the strongest outreach group is very vague, and they are very good about being anonymous and careful when contacting people. If I were to see a sex worker, and was concerned about them, I would pay them, and give them the information. You can't save people, but you can let them know about services designed to help them... you can let them known that they are not alone.
Although, for the woman I was talking to, one of her pimp's main goals did seem to be to isolate her, and to shame her for trying to be educated, so who knows what repercussions there could be for looking for help, especially for someone in a worse situation. Maybe contacting the local sex work outreach groups and letting them know you saw a girl you were concerned about?... I'm not sure, they don't really like to talk to johns. It's very hard to know what to do.