Usually, it's not hard to check an spg without touching a thing.
Unfortunately, I've had dive masters do this on a dive as well. My very first dive on the Castor out of Boyton Beach, the boat had an affable Dive Master who was not certified as such. I dove a 120 and he was in an 80. He signaled that he had a lot of gas near the end of the dive, which impressed me so much that I was concerned. I dipped below him and looked up at his SPG which was showing way less than 500 psi. When we got to the upline, I sent him on up and took the task of unhooking us from the boat. On the boat he has bragging about having over a thousand psi left. The Captain is a good friend of mine, and after we docked I carried the tank into his tiny office there and put a gauge on it. The Captain was not only shocked, he was pissed. There was very, very little air in the tank.
Why did the DM lie? We say that Scuba is not a competitive sport, but it really is. We all want the most, the deepest, the longest, to use the least amount of air and so forth. His antics put all of us in a bad way. What if he DID get hurt while trying to undo the anchor line? It was our first dive of two so the entire trip would have been cut short.