You could try carbonating a keg, then turning off the gas at the tank and disconnecting it from the keg. If your beer doesn't pour the next day, the leak is at the keg.
A long time ago I saw somebody took a section of an old inner tube and fit it over the top of a keg so he could fill it with water and look for bubbles. I thought that was pretty cool, but usually spraying with star san works well.
Yes, regulators can leak.