If you see anyone underage being prostituted, it doesn't matter if they are flirtatious, seemingly willing, addicted to drugs, angry, sad, happy, whatever...doesn't matter. They are a trafficking victim if they are being sold for commercial sex and under the age of 18. That is federal law.
Also if you see evidence of pimp control no matter what the age of the person being prostituted, the one being prostituted is also a trafficking victim. So, it doesn't matter if they are addicted to drugs, hard talking, rough, flirtatious, seemingly willing, etc.
Make the call, save lives!
In all cases, show compassion and kindness. Almost every person who is being prostituted has a history of having suffered extreme trauma and violence. So even if you don't suspect human trafficking in a particular instance, remember, these are our fellow humans. Treat them with respect.