I don't really know how you can tell if an ad is done by an undercover unit or not.  If you stick with well-reviewed providers whose reviewers are established with many reviews themselves, you should be OK.  Ad sites like backpage and others are generally not good places to look at ads.  However, if you do find someone on one of those sites, check here and other review sites to see if she does have reviews.  If not, move on.  You can also Google the phone number, and use tineye or google picture to verify her pics.

Swim

You have to scroll down & read the threads.   Your education in the hobby is your responsibility...  there are no guarentees & no short cuts.
Well Reviewed providers are the safest bet...  and are often recommended for Newbies.
Stings are local so would not be posted on a main board...  but the nature of stings is such that LE  sets them up, uses them a couple of days & then disappear for a while.  While bigger depts may have dedicated Vice Squads, a lot of smaller depts use one set of Detectives...  maybe borrow a guy from a neighboring jurisdiction but then they go back to other detective work...  unless they get a complaint.
This hobby is all about being discreet & keeping a low profile.  Do your homework & you'll find much written about it.
skb

If there was a surefire way to know which ads were LE , etc. then LE wouldn't exactly be doing their job. If it was a simple surefire method we wouldn't have to worry about screening, etc.  As a provider, I know certain items that come through to me that are suspicious but that is my little secret and I'm sure not putting it out on a public board. I know certain areas, ads, wording, procedures that are more prone to be associated with LE.

As another poster mentioned, even with all the tips and advice in the world, it is ultimately your responsiblity to protect yourself

You're playing with a lot of risks in this hobby - that is part of the allure of this hobby.  However, there are ways to mimimize your risk. So read posts and newbie manuals and listen to seasonsed hobbyists. Because I'm telling you from experience of many, many years gone by....once you are naked and alone in a room with 6 vice officers it doesn't matter HOW they found you..it is too late....you just better STFU.

If a provider is willing to have you come over without the least bit of screening, that could be a decoy luring you in.

If a client is so anxious to have you come over right now that he'll pay you double your regular rate, that could well be LE trying to lure you in.

LE normally sets up a sting on their own turf, so if you are a client going to an incall, or an escort going on an outcall, beware these lures.