Look at the case. If it's split, or deformed, the chamber has wear or has been damaged.
Same deal if soot on the side of the case, it's not sealing.
Without a cartridge in it, look at where the bolt handle locks the action shut. See if you have any slack at the locking lug, by pushing the bolt forward with your thumb. You should not be able to create a significant space there. Often it will split the case around the edge of the rim, if this is too loose.