Originally Posted by Chrome Prince

Stick close to the mounting yard and look and listen to the trainers instructions.

Often you will hear things yelled at the last minute before they do their preliminary to the barriers.

Things such as...
"There's a prize for fourth remember" = no bet
"Be in a good position at the turn" = bet
"Keep him out of traffic" = bet
"Do your best" = no bet
"Watch Jim's horse" = bet

Make mental note of the strappers and trainers.

See if they are betting on their own horses in the ring.

Look at the faces of the strappers leading the horses around, a lot can be gleaned from the smile and friendly approach to the crowd, a pep in their step. As opposed to solemn faced, not talkative.

Just a few clues.