This is a freelance model's main way to "prove" their skills in the industry. If you've got a good collection of images from shoots you've done that are impressive and showcase you in the best light, it's enough to justify getting paid.
In addition to having great images in your portfolio, if you've gotten lucky enough to obtain tearsheets from work you've been published in, you're definitely within your right to charge if someone wants to hire you
It's typically easy for clients to know when they're dealing with someone that is professional because they'll carry themselves as such (please note: acting like a diva doesn't count!) Your Comfort Level with Your Skills: Confidence in yourself goes a very long way in being successful in modeling. Are you still unsure of your skills, feel a bit uncomfortable in front of the camera or think you still need a bit more practice with doing shoots?