With CO2 the gauge doesn't tell you what's "left in the tank" really. It will stay at a constant pressure (at a particular temperature) until the tank runs out of liquid CO2 and then drop rapidly. If you had a leak bad enough to start dropping your tank pressure at all within that short a period of time, it would already be practically empty in the time it took you to write your question.
500PSI is quite normal for fridge temps, as is 800 for room temp. Sounds like you're fine.
It's still a good idea to check for leaks no matter what - some starsan or soapy water in a spray bottle makes it really easy, just spray it on any area there could possibly be a leak, and look for bubbles.