you can use a level but the easiest thing to do is to use a ball and roll it here and there, try setting it up in a pull, push, pullkick, pushkick on the 3 rod and 2 rod...hand roll it slowly from along the 5 bars, etc, etc
you should be able to tell the difference between the table itself being level and the surface...you may want to try and level the table first using a level and getting the feet right - this takes 10 to 20 minutes depending on the table, surface it sits on, and if you have help
with that said, the only warped surfaces i have fun into were the middle to late run metal wrap T2000's...none of the previous models ever had a manufacturing issue with this so it would take tons of moisture over a long period of time to warp an earlier model
no easy way, but first ask, then test