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

I have a TPMS with sensors on my trailer and on my dually truck. I had a flat on the inner tire and only knew about it because of the monitor. It did not happen while I was traveling but I got an alarm as soon as I started to move the truck. I could not tell by looking that it was flat. I use the system even when not pulling the trailer for that exact reason.