As
WyoTraveler mentions, you can thump the tires to see the one that is running flat, or you can purchase an Infrared Thermometer from
Harbor Freight. Makes it very easy to check all 6 tires, not just the duallies.
Whenever we stop for gas, eating, or rest stop, I take a quick simple walk around. I aim at the tread about 6" off the ground and keep a mental note of the readings. The highest temperature is the tire with the lowest pressure. If it is more than 5* degrees above ambient, I get out the tire pressure gauge and double check.
I did stop in Nevada once and one side of the RV was warmer than the other because of the direct sun versus the shaded side. If you wish, you can take the temperature reading of the side of the RV and compare it to the tires on that side.