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

Thanks. I didn't realize how simple the tire pressure monitoring systems are to install (I was imagining something inside each tire). I notice they can be had with five sensors - which would be nice, because getting the cover off the spare is a pain, and it would be a good thing if I have to do it only when I have to.

Folks here are so knowledgeable. Thanks for your generosity in spending time here explaining and providing info.