In poker, the loopholes of each player are called leaks.  

• Against players who play too many hands, play fewer (better) hands
• Against players who play too few hands, raise them preflop to win the blinds from them
• Against players who never bluff, always believe them when they bet or raise
• Against players who bluff too much, call them.

It is actually easier to identify these players when playing internet poker, than it is when playing live poker.  You can use your computer if you want to, or pencil and paper..

(from The Theory of Poker, by David Sklansky)

Pluck the pebble from my hand, Grasshopper.