another good check is to put the cb on the spot, shoot at center diamond and have cb come straight back to your tip. if your not straight you'll see it quick.
Dr. Dave labelled this the "Most Famous and Useful Drill of All Time" (MoFUDAT).
You can also do a couple of variations:
1) Put a target ball in front of the far diamond/spot, a hair or a little more off of the cushion. The goal is to shoot the cue ball into it, have it rebound back, and return the cue ball as straight back as you can. I always considered it pretty good, if you can even get within a diamond on each side of the center of the close, end rail.
2) Put the target ball a diamond, or wherever really, on the same cue line. Shoot a stop shot, and have that ball go down table and return to the cue ball.
Obviously, if the balls are not set up perpendicular to the cushions, it will throw the shot(s) off, but hopefully your address at the cue ball will compensate--experience also helps, but if you're drilling with these, then be wary of getting frustrated!