If it is as you say, the same hot water pressure issue at all faucets but cold is fine, then I'd say you are starting at the right place... the water heater.
You didn't list your specific make/model of water heater, but yes, there is almost always one (and sometimes two like mine) check valves which are on the back of the water heater. Sometimes easy to get to for service, sometimes not.
First, I'd check all my bypass valves, any exterior showers and such to make sure all the valves are set properly as Edd505 stated.
Then, here's what I'd do to start narrowing down the issue... 1) turn off all gas/electric to your water heater, turn off the water supply and allow heater to cool down. 2) Remove the drain plug on the heater and let it empty. 3) Turn on the water supply and see how much water comes out the drain plug. If it's a trickle, then there is a problem with the water supply to the heater. If there is a solid stream, then the heater is getting water and the problem is either the check valve on the output side of the heater or further downstream in your plumbing.
Ed & Joie
2004 Vectra 40KD
Freightliner Chassis and Cummins ISC