When I'm replacing alternator belts, ac belts...etc, how can I tell is the belt is tight enough?

 

measure the deflection of the belt by pressing the belts at the midpoint with your thumb with n lbs of force. Look it up in your helms or chiltons or haynes... usually it's anywhere from 1/4 to 1/2" of deflection at about 20-25lbs of force.
Personally, for the ac, ps, alt belts, i just guessimate how tight it should be and if it doesnt sqeel or whine thats good enough for me