To pressure-test, read this Tanklet: http://www.waterheaterrescue.com/pages/WHRpages/English/Troubleshooting/Tanklets/thermal-expansion-TP.html
Pressure shouldn’t be greater than 80 psi at any time. As to expansion tanks, they need to be charged to line pressure to work properly. They come pre-charged to 40 psi, but if line pressure is higher than that, they need to be pumped up.
Also, the rubber diaphragm inside them can fail with time, especially if the tank is mounted above the heater in such way that hot water flows up into it. Mounted elsewhere, cold water will flow in.