How do I know a tire's low/flat on dual wheel axle?

BillHoughton wrote:
Today's dumb question...

I check tire pressures before our trips, and during trips (though not as often as I should during trips; but certainly before long drives).

But how do I know, when driving, that a tire on a dual wheel axle is low or flat? Obviously, if I hear a blowout, that's a clue; but what about more subtle flat conditions, like a nail in the tire?

Bill,
Several have mentioned IR meters. They are talking about using an infrared thermometer. When a tire runs low on air it heats up because of the additional flexing. We bought one of these - Clickable Link
They can be bought for less than $20. Whenever we stop, I grab the Infrared Laser Thermometer aim it at the side of the motorhome to get an ambient reading, then aim at the 3 tires on that side. Then I do the same on the other side. The tire with the highest temperature is the one that is lowest in pressure.