Most guys with some experience know this but newbies might not. In many cases, a lady having a driver can be a clue about a pimp. Certainly there could be non-pimp drivers though, so like with most of the clues, it's best taken into consideration holistically, in combination with any other clues you may have noticed, and not as your sole basis for evaluation.
The lady you wanted to see is not available, but her friend is - the day is saved!
Some ladies do have friends they will pass off appointments to if they are ill or unavailable for some other reason. Less common is if the lady you originally wanted to see takes care of everything and practically makes the appointment for you with the lady she's recommending to you rather than just referring you to her friend. That might be a cue to start thinking along pimp lines.
If a provider you've never seen before reaches out herself promoting another lady to you, to me that is strongly suspicious and suggests pimp involvement. It might be a little less suspicious with a lady you've seen before, she could just be helping a friend get started. I'd still be a little wary in that case unless it's someone you know very well.
If you book with one lady and another one shows up instead and says her friend became suddenly unavailable for some reason so she came instead, that's a strong indication of a pimp.
If you see the same person posting the very first review of nearly every new provider joining here - yep, sure seems likely to be a pimp. Particularly if it's a user with a low post count and recent join date. They are also generally careful to clearly explain the upcharges in ROS, for your convenience.
Shill reviews help their ladies get verified, are a great way to promote them, and as a bonus, also gets the pimp free premium access, so he can monitor both the Men's Lounge and the Powder Room (from the provider sockpuppet accounts he operates).