American Pharoah Jockey -- Horse Betting Tip ... Consider the Penis

Ever notice those degenerate gamblers checking out the horses before the race?

Turns out, they're looking for one HUGE tell-tale sign -- because according to Triple Crown winning jockey Victor Espinoza ... the key to victory is all in the horse genitals.

"You wanna see if the horse has his penis out, then that's the horse." Espinoza tells TMZ Sports.

So ... a horse is a horse, of course, of course, unless of course he has a massive horse boner .... then he's a WINNER.