If you've got a piece of very flat concrete about 2 feet square (checked across both diagonals with a reliable straightedge) you can park the mower on it and see if front and rear rollers are both in contact with it at both ends. If they are not, it can be twisted back into alignment but you want to have it sit very flat indeed, without prestress. If it is prestressed it will creep back into a twisted state sooner or later. Use a feeler gauge or something similar to check that all four corners are touching. Remember, that piece of concrete has to be as flat as surface-ground metal.
