If you have access to it, watch the refs signal on the sideline. Hands crossing over the head, stop the clock, arm swirling like a windmill, the clock runs.
Now I don't remember the play clock running rules, but I believe that the rules changed about 10 years ago where even if the runner gets out of bounds, the game clock still runs except:
Last 2 minutes of 2nd quarter
Last 5 minutes of game
Change of possession