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

After driving you can check the temperature of the tires with your hand (carefully) or with a laser IR thermometer. A low tire will typically be warmer than the others, though some differences in temperature are normal and expected. (The inners are often a bit warmer as they have less cooling airflow, and the right inner on my motorhome is also warmer because it's closest to the exhaust system).

If you check the pressure before each day of travel, you'll be well ahead of many people already.