Slightly awkward question... A pressure gauge is good and useful, especially having a pump with a gauge, but some air escapes at each measurement, so I wonder if there are other methods that give reasonably good estimate.
The reason for asking is that one of my bikes is a Brompton, the small tires contain very little air and also the valve is a bit hard to get at because the spokes are so close. So when I measure the pressure with a gauge, I lose some air just from the measurement, at best perhaps a around 0.3 bar, but if I don't quite manage to push the gauge in properly, it can easily be 1-2 bar (it only takes a second for most of the air to escape). Also pumping it up is hard (especially when travelling without a big track pump), so I want to avoid losing air.
Normally I just use the "can I squeeze it with my finger" method and the "look at how much the tire bulges when I sit on the bike" method (and then a pump with a gauge occasionally when it feels too soft), but these are not very precise.
Basically these are hardness tests for the tire, so I wonder if there are any (light, cheap) instruments that can measure the hardness of the tire in a noninvasive way.
Obviously, relating the force to the pressure depends on the tire material, so it wouldn't be calibrated to pressure, but I'd be happy with a result like "the tire is softer than a certain threshold for my particular tire that I defined earlier".
My tires have a pressure range of 4.5-7.5 bar, and I usually pump them up to 7 bar. So I'm not too worried if it's 7 or 6 or even 5.5 bar, but it shouldn't really be below 5 bar.
Update: I also experimented with a cheap valve cap pressure monitor and wrote about my experience in the answer to a different question.