Start with the mower sitting on flat level spot with proper pressure in all 4 tyres
Raise deck to second highest cutting position
set both blades across the deck.
They should be exactly the same height where they come together and at each end.
rotate both blades one at a time.
If the difference between the two is the same every time, deck is bent , broken, split.
If the difference changes then blade or spindle is bent.
If both blades are dipped to one side but come together in the middle then the deck is not hanging level