In brick-and-mortar game, physical tells are decisive. Sweating, breathing, eye pupils and their movement and volume
(eye pupils are the organ that increases volume 6 times during excitement - and not the organ you'd think of...)
also, the direction of sight, any attempts to hide mouth with hands, any clenching of fists, the blushing of face, the famous gesture with crossed arms "I am disinterested", etc...
In online game, look if the opponent used "check/fold" automatic option on the previous street. Then, a high card appears and he suddenly bets big...
Also, monitor the betting behaviour - what amount relative to BB does your opponent use to raise preflop. If he typically raises 3 BB and after 1 hour of play he raises 5 BB - be careful...
If no action happened on the flop and turn, and if the river does not complete any str8 or flush - be ready to call a big bluff...
Well, I may be disagreed by some members here, but this is my point of view...