The easiest way to check is to open your lower faucet on the water heater (with a hose attached) and see what kind of pressure you get out. If the bottom of your water heater is full of sediment, you will not get much. Another problem with water heaters is the dip tube, if the dip tube has lost the lower half of the tube, (dip tubes are on the inlet cold side) then the cold water will mix half way up the water heater and you will cool the water immediately. The dip tube positions the cold water at the bottom of the tank so its closest to the burner and does not disturb the hot water at the top. There was a problem with the dip tubes on many water heaters due to a manufacturing defect. The dip tubes were made out of a material which broke down after a few years, some of them causing this exact problem.
Just an idea.