I detect it first by the change in the color of the smoke..the thin blue smoke becomes a darker and thicker gray smoke. As the temp continues to ramp up, I then see heavy "puffs" of smoke where a puff will appear, then no smoke, and then another puff, etc. Sometimes it ends there and I conclude the combustion has been avoided. If combustion does happen, then the final observation is a huge "belch" from the smoker...like a shot of hot air escaping from the top and around the door. When a remote temp probe in the smoker, you will also noticed a spike in the temp within the smoker. After that, then the regular smoke starts and everything returns to normal.
If combustion is going to occur, it will happen within the first 45 minutes--for me, it is usually when the smoker hits around 180-200F.