About the cops posting: I'm sure it's legal, but I don't know how much they do it. I doubt they want to deal with accusations of entrapment.
The elements of the charge of soliciting for prostitution are an offer of a fee in exchange for sex. Sex, for these purposes, is touching someone for the purpose of sexual gratification. By that logic if you wash the feet of a foot fetishist, that's sex, but I dunno if they carry it that far.
So a cop will try to steer the conversation around to you offering money for sex. Genuine providers will be much cagier about it; they'll speak in terms of "sessions" and "appointments" and usually avoid mentioning a fee for sex. So if you're talking to a provider over the phone and you get the sense she's steering the conversation that way a little too much, maybe you should get cautious. Experience should give you antennae that wiggle when the conversation starts sounding a little odd. Never forget the police are allowed to lie. A decoy is free to lie if you ask her straight out if she's a cop.
I wish I could just give you a 1-2-3 formula, but there isn't one. Experience is the only teacher. The good side is the chance or being arrested, wile not zero, isn't high, either. Solicitation is a misdemeanor and so not all that rewarding for the cops. Actually, when the cops bust a place like a spa, the men they find there are generally let go after a check for being wanted. The cops aren't trying all that hard to shut down the hobby, (personally, I think they just want to keep it from getting too large or blatant), with one big exception: the street.
The street scene is where the police spend most of their effort. There's a lot of crime and quality-of-life complaints surrounding the street, and that's where most of the effort goes. Clients do get arrested on the street and their cars are confiscated. Police often use decoys, female undercover officers. Two danger signs are guys hanging around (generally in cars), and situations where there are only one or two girls, which are the decoys (the cops usually take the genuine SWs off the street when setting up a trap). Decoys move around less than the real SWs, to make it easier for the backup team to protect them.
When a client or a provider arrives at an appointment, the usual drill is the client puts the fee down on a nightstand or suchlike and the provider picks it up without a word. How effective this is is open to question, but at least nothing incriminating is on tape.
To sum up, the risk is real, but it isn't very high, certainly not as high as some people would have you believe. Look up the thread "have you ever been busted?" for poll results from members here. About 5% of the respondents have been arrested at least once, and that poll included providers.
Be discreet and pay attention, but don't go around in terror, either. It's a little like working with electricity; understand the risk, but too much fear makes something going wrong more likely, because you miss real warning signs because of all the imaginary ones.