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

I've felt a flat dual on a truck as an imbalance, just like a tire that was out of balance.

When stopped (fuel, break, ect) I'll reach under the fender and just push sideways on the inner tire; if it gives way it is low on air. Or you can stand behind the truck/trailer and look for a tire that has an excessive bulge when sitting on even ground.