use pokerstove.

If you havent got it google it and download.

Its basically a good program to help calculate your ev.

For example, you have AA on a low board that contains 4toaflush. You bet opp pushes all in after you made a standard preflop raise in mp and the button called.
Hence you input your hand the board and opps range and simply calculate of you have the pot odds/ev versus opps supposed range to call (assuming opp is not a nit only pushing a set here)

Ive inputted a lot of what are now 'standard' situations versus different players into it and calculated where i should be making calls and where i should not.