Peter is right, a police decoy will not get into your car as there is too much of a chance of losing the car if you decide to take off. However, the downside is if you pick up a very strung out girl, she's already in your car and your stuck trying to get her out.
If cruising the strolls, best thing is probably to park your car a block or so away and walk back to any girls you think likely. This gives you a chance to get a good look at the girl, and to decide if she's worth your money, and also to decide if she's too strung out to risk. This also gives you a chance to check out vehicles in the immediate area for police backup if she is a decoy. The backup will have a line of sight on their decoy, and their vehicle may have extra antennas. As well, if you watch what you say to the decoy (let her make the suggestion of money) you can walk away afterwards. Even if the cops try to get your name, you don't have to give it to them unless they arrest you. If you're in a car, you have to provide your licence and registration if asked by police. They do have to have a valid reason to pull you over though, as Calgary Police had an impaired case thrown outa few years ago because the only reason the guy was pulled over was for cruising the stroll. (The case was Regina Vs. Bell, if interested.)