There are two ways. Feeler gauge is the best, but it can be pretty quick and dirty. Hold a string down at the first fret, find the blade the fits under the second. Lets say its 3 thousands. Now jus move up the neck with the 0.003 blade, you will be able to tell if its getting tighter, looser or staying about the same. Remember that if the string didn't buzz at the first fret with 0.003 (or whatever) gap, it won't buzz at any other fret with that same gap. The really quick test is to hold the string down at a given fret and tap it on top of the next one - you want to feel a bit of movement and hear the little "ping" that tells you you have a gap. It doesn't tell you how that gap is changing but with a little experience you'll learn when you don't have enough.