It is appropriate to do a quick vehicle inspection on occasion. It would be good to know if a tire needs air. Is there a way to know without having to use a tire gauge?
Thump them like truck drivers do with a small bat or hammer. They will make a different sound if they are low, you can also look at how squat they are. Look at the tread and how it is wearing. Over inflated tires will tend to wear quicker in the middle, under inflated tires tend to wear quicker on the outside of the tread.
Gauges are cheap typically less then five bucks for a simple one that will do the trick, and also most places you can get air have a built in gauge, although these gauges are notorious for being inaccurate.
A gauge is best, but gauges on gas station pumps are often inaccurate.. Looking at low profile tires can be deceptive.
By pressing on the sidewall with a thumb, I get a better sense of pressure than by looking, but accurate only to ~30%. Kicking the sidewall (gently) can be used to make a tone, similar to the hammer, but I wouldn't expect better accuracy than by pressing.