With illegal prostitution there's serious hazzards and concerns about females working against their will, or forcefully have to give off a great portion of their revenue. You as a client is able to report these practices of illegal activity based on a few key signs that we've learned over the years working in this business. It is no fun to encounter women against their will or active sex traffickers who push people from one to another country with the intention to have these work in the prostitution business. With reporting you can make a difference.
This business relies on eyes and ears - and without it officials have a hard time finding out or even having an idea of what is going on. You can note illegal prostitution or forced prostitution even by very clear signs. Think of advertisements where all services are offered for a duration of 24 hours. No healthy women in the world advertises herself for 24/7 available and esp with as much as possible service. Additional women under stress, always needing more and more money, or people who seem to put everything in motion to get girls moving.
Hauling these in under false conditions is also a form of illegal prostitution. Think of one claiming a job offer that is just too good to be true and ending up likely in sexwork under harsh conditions. Please call your local authorities or even report crime anonymously. Don't hang out to be a hero - this can be a dangerous business in some countries.