Before taking your tank to have it refilled/swapped for new. Look all over tank and find what the empty weight of the tank is then weigh the tank and subtract the 2. If it gets down to around a pound to pound and a half, time to make plans to have it refilled. If you dont have a scale with that capacity, there is a workaround. Get your bathroom scale (the one your significant other yells at every monday morning) and weigh yourself. Add weight of EMPTY tank to your weight, step on scale while holding your bottle and subtract the 2. This will tell you how many pounds of gas you have left.
Also, when stored inside the kegerator, your high pressure gauge will drop to around 500 psi. This is normal, when you notice it dropping lower than normal levels, its on its way out. Mine hovers around 480psi. Soon as I see my high pressure gauge drop, i plan on going that weekend if I am going to be charging a new keg, if not I will wait till keg empties then have it refilled.
You had also mentioned that the high pressure side still held pressure a length of time after you had realized you turned the gas off. This is a good thing. This means you have no leaks. Had you had any leaks it would have dropped significantly.