I have lost hot water pressure in my entire house. The cold water pressure is fine. We have an electric hot water heater.

Test the pressure coming off the drain spigot. If that pressure is good I'd suspect the heat trap or nipple at the top of the tank.

The tank may have a hole in it

Is there a tool to use to test the water pressure? What sort of pressure is good pressure? How do I check the heat trap or the nipple? Thank you so much for your help.

Just open the spigot and direct the water into a 5 gal bucket. You should be able to tell if its better than in the house just by the way it drains, do not close your main supply line. Your pipes above the heater: copper; galvanized, etc.. What type of connection is used to the tank; solder, coupling, etc...