An easy test to check the dip tube is to open the drain on the bottom of the tank, open it quickly and open it a few turns, you want good flow. If the dip tube is good, you should get cold water after a few seconds , if the tube is bad, you won't get cold water.

The dip tube brings the cold water to the bottom of the tank where it get heated. If it's faulty, the cold water goes in at the top of the tank, then out the outlet line, giving warm water instead of hot water.