I've never actually picked up one of the girls that work the casino but I've been approached a few times and also tried to figure out if a girl is looking so here's a few things I've noticed.

Look for girls that are dressed nice, sexy in an understated way. They will be by themselves, most girls that are out for a night on the town in a casino and going to the club are either with a group of girls or a guy. That doesn't mean every single girl would be working but it helps narrow things down.  

If the single girl who is dressed nicely is just sitting in one place for a long time then odds are she's just waiting for someone and not working. If on the other hand she's moving around every few minutes then sitting down and just messing with her phone or looking around there is a chance she is working.

I've been approached by 3 girls in the Venetian and Aria and every one of them matched this description. I've spotted numerous girls like this and watched from a distance and most ended up going over to a single guy and chatting him up. Every time it was someone that was older and would probably have no chance with a girl like that who wasn't working.

Two nights ago I was in the casino and bored so I decided to start cruising around looking and I spotted a girl matching the above description. I walked by her a few times, glanced at her and then finally when she sat down again I played a machine a few down from her. She looked right at me, gave me a very sexy smile and wave. I smiled back and moved on but I'm sure if I hadn't moved or sat next to her she would have offered.

Again all of this is just my observations and I could be wrong but it has been pretty consistent over time.