I had a new water heater installed recently. Since the installation, the pressure relief valve has been leaking. The installer tested the house water pressure and said it was too high. I just had a new house water pressure regulator installed @ an even lower water pressure than before. The old water heater has never leaked prior to the new one being installed. I don't know what the pressure was before but I now have it set @ 56 PSI. And that is less that what I had before. Is it possible the pressure relief valve on the heater is faulty? How can I check the water heater pressure ?

Valve could be defective
OR
A back flow prevention/check valve was installed with your new Water heater
OR
You need to add a expansion tank in the cold water line between the backflow valve and the water heater. These tanks give the extra water a place to go.

Do a Google on your problem to see what the items above do to correct your problem.

You stated that you have a new pressure regulator, and most have a check valve in them. If you don't have an expansion tank on the cold water feed side of the water heater, the hot water expands and it has no place to go... but to start lifting the pressure relief valve off it's seat and it starts seeping water. This is real common with the new water meters that most cities are now installing... you can go to many of the better HVAC / plumbing wholesalers and ask for a tattle-tale water heater drain pressure gage. Just screw it on your drain valve, open the valve to see your tank pressure, then set the red needle to the current pressure. As the pressure increases over a time... the black needle will move the red pointer to the highest pressure found in the tank during that time frame.

As well, you can go to the following web site Gas detectors | Thermal Imaging Cameras the have water pressure gages that I have described above... great site, I purchase a lot of my inspection / testing gear there.