Testing a pool cue to see if it's warped is one of the easier tasks for a pool player. There are a few different ways to check, but by far the most popular and easiest way to see if your cue is to "sight" the cue. Hold the cue up at eye level and look down the cue from the butt. Hold the cue out and slightly downward, slowly turning the cue as you look down the cue from the butt to the tip.
Some will tell you to "roll" your cue to check for warping, however this is not the most reliable way to check. In order for the roll test to work, you need to find a perfectly flat surface. Pool tables are not ideal for this sort of test. Even the slightest variation in cloth or table felt can cause your cue to wobble a little when you roll it, even if your cue is straight. Sighting your cue is the most reliable way to check for warping.
Whenever you buy a pool cue, the first thing you should do is sight it to make sure it isn't warped. If the cue is to your satisfaction, chalk it and play with it, but if you notice a significant warp, call before chalking the cue in order to have a replacement sent out.
It should also be noted that because pool cues are made of wood, they will all warp over time. Some manufacturers like McDermott Cues offer lifetime warranties on their cues against warping, so if warping is a major concern check the warranty before you buy.