Well it depends, I found that those who are serious often got notes by the entrance, saying that they are strictly non sexual. It's many things that can be checked beforehand, if they have a website that looks serious. If they have both male and female workers, reviews from other people online. Those who are brothels in disguise, is very often not registered as a company at the tax registrar.
But honestly, one can never really know. Its a lot of thai massage studios that are brothels in disguise. Nigerians often uses hair salons as a disguise.