I'm assuming the car is up on jackstands? Pick seven or eight points on each side that are symmetrical (K-member mount points, rear LCA mount points, gas tank strap points, etc.) and using a plumb-bob, mark the locations on the floor. Then break out a tape measure and start connecting the dots (K-member front left to LCA mount rear right, etc.) measuring the distance between them. If you're getting substantial differences in the cross-dimensions, then you're bent. Before you toss the tub, you may want to run it to a frame shop and see what they say prior to making a final decision. These tubs are tough, and unless something is creased, it may not be as hard as you think to straighten the thing out if it's bent.