If there's a clear "Table 1" type of table (i.e. one that's a little nicer than the others, near the bar, front and center for spectators)... and lots of tables are empty and someone is playing on it, I assume they're one of the house sticks and therefore possible gamblers.
A guy sitting at the bar with his cue case, not drinking and not shooting either. He's looking out at the tables and not at the bartender/tv... casually eyeing everyone who comes in and watching them shoot.