Use a leveling tool on a flat surface.
A model number would tell what deck you have.
on the mower deck
I have a Craftsman DYS 4500 and there is a nut under the left rear fender next to the tire that adjusts level. I'm still having trouble getting the deck tuned again despite my best effort with a bubble level, but that nut is definitely adjusted it.
john deere LT155 38inch cut mower deck
you were no help
http://manuals.deere.com/cceomview/OMM143122_E0/Output/Index.html?tM=HO
Measure the width of the deck with a tape measure.
38'' deck
park mower on level ground or concrete. lower deck all the way down to ground. lay on your side on the right side of mower. several locking pins need to be pulled out of their slots. lay these to the side and do not misplace them. one cable and washer on top of deck need their locking pin removed. second locking pin needs to be removed from same cable guide way toward rear of deck. this makes 4 pins in all from right side. lay on left side of mower on the ground. three pins on this side need to be remove in same manner. raise height lever all the way back up and this will make it possible to clear mower when you slide deck out from under mower to work on deck . after work is complete, slide deck back under mower as close to location it was in when you started. connect all parts and reinsert pins in reverse order of how you took apart.
See sonnysbolens.com for free online manual reyour mower.
How do you change a mower deck belt on a John Deere L130?Read more: How_do_you_change_a_mower_deck_belt_on_a_John_Deere_L130
In most cases removing the deck will make the job easier and faster.