Personally I would just replace the old water heater myself since it's already way out lived it's normal life and is most likely full of lime and calcium. Having said that it is very easy to check the manifold pressure on a water heater at the provided plugged pressure port. In fact you can make a water column gauge with a piece of 1/4" plastic tube marked in one inch increments from the bottom and a tall bottle of water. Like I said, I wouldn't even bother on a WH that old.. Most all newer residential water heaters do not have a PRV but are designed to operate off of normal house gas pressure.
By the way, Stretch and Turtle both know what they are talking about, the other troll just likes to type the "F" word and cause trouble all the time.
I've been do> I posted a few days ago about my high gas bills and I still haven't had