You assume cops need evidence lol.

Jokes aside, stings are the most common way it happens.

However, when it isn't something like that, it tends to fall into two groupings based on my limited area of awareness. Not a cop, not a lawyer, or anything related, and I can only guarantee this as factual in the areas I've lived.

The next biggest chunk of arrests after stings is warrants for places of business. Massage parlors, more blatant brothels. Once there's suspicion of illegal activity, they get a warrant an d raid the place. Catch them in the act, and then manipulate the people they take in into confessing.

After that, it's plain sight and/or hearing. While most of the street corner type of thing is gone in a lot of places, it isn't totally gone. All it takes is the discussion over prices and seeing the exchange to make an arrest.

Plus, some places will post up with listening gear or cameras in hot spots and capture evidence that way.

The kind of services that are less open get less raids. An escort service is usually harder to nail down than a massage parlor, which is harder than an outright brothel, which is harder than someone working out of a hotel or home. The really hard to catch prostitution is outcalls and incalls with independent sex workers.

Escorts don't do any transactions in the open, so you have to send in an undercover cop to fake being a john for individuals, or get enough probable cause to get a warrant for phone or other invasive surveillance when there's an office.

Massage parlors have that veneer of a massage business, and if they aren't doing it dumb, they can last years before police even know they exist in a big enough city.

Brothels, afaik, are pretty rare now because they're easier to target. Since they're really only serving sex, it's more obvious.

Solo workers tend to have to go out to find clientele, which is obvious enough to get attention. Kinda hard to advertise yourself enough to get clients without cops being able to tell, and then hassle the worker. If they're working out of a bar, it's a little easier to hide their intent, as long as the bar is okay with it. Some places take a cut to ignore it. One of the few regular bars I worked at had a couple of ladies that had made arrangements like that with the owner. Iirc, they were handing over ten percent and getting the benefit of being inside and not hassled as long as they didn't cause trouble.

At the titty bars I worked, some of the ladies would turn tricks with patrons. They only did it after their shift because the owner was not willing to fuck around and lose his licences over it. They'd get fired in a hot second if they did anything other than say they'd talk to the customer after their shift. None of them ever got caught. Hell, one of them turned tricks with a cop or two, which helped in that regard.

But, yah, the vast, vast majority of arrests are stings. It's low hanging fruit tbh. Why spend resources trying to get evidence other ways, when it works and keeps the segment of sex workers most likely to cause disturbances more limited? Escorts don't cause trouble usually. Massage parlors don't until they get too big and busy. They want the obvious hookers and blatant shops out of commission. If it isn't visible, it's lower priority.

Now, the sheriff of our county handles it very lightly. Pretty much all sex work out here in the boonies is drug related to begin with, so that's the key to keeping things controlled. Much, much easier to bust a prostitute for meth than for prostitution. So he's got an unofficial policy against targeting prostitution as its own thing. Which, that's its own problem, but he's kinda stubborn about drug policy other than weed.

There was a place that got busted in town a few years ago though. It was a ratty little place that was billed as a massage parlor. But all of the workers were addicts, so they were sloppy as hell. The lady on the phones was even stupid enough to discuss pricing for sex acts on the phone.