good way to level your mower deck?

I have a 54 inch mower deck on this 425 John Deere that my neighbor gave me. Just got the deck on it the other day is there a good way to see that it is level before trying to mow? Do I need to get the ht adj first then lower or raise my gauge wheels? I don't have a book on this but I think I will try to find one.

Thanks Gary Mac

good way to level your mower deck?