Cartridges that I have seen, which have had a gas leak occur, had a black streak, similar to soot, on them. If they don't have that, then I would be looking for something else. A bore light might help, but casting the chamber, after it is thoroughly clean, would be the best bet. You can measure it for concentricity, correct diameters, etc., along with seeing any imperfections.
I need to add, that it was an AR that I saw this happen with, over at the range one day. I didn't notice until the shooter was gone, and happened to see some of his brass laying around.