Well, based on what you said, it probably does a DEC on the amount of lives, then does a BPL or a BMI to determine if you have a game over or not. If you are at 128 or higher, the negative flag will be set, and the BMI will trip, causing the game over.
As one alternative, you could do the DEC, but then branch if it's zero or not (BEQ/BNE). This still won't be a perfect check, since the game probably doesn't have any bounds/wrapping check on the number of lives. I don't know this for a fact, however.