Can I estimate my road bike tyre pressures by pressing my thumb into the tyre? I don't have a tyre pressure gauge, and feel that the tyres on my new Merida Scultura are pumped too hard.
No, not really. It is difficult to get an accurate idea of tyre pressure by pressing it.
Especially for thin, high pressure road bike tyres - they should feel pretty hard. Even if they do feel hard, the pressure may be much lower than it should be.
Best option is to get a good pump, with a built in pressure gauge. Many mini pumps will take a lot of effort to get the pressure high enough, a 'track pump' would be a lot easier.
Tyres do lose air over time - especially thin, high pressure tyres - so it is a good idea to check them regularly, and inflate as required.
you should get a track pump, this will be very useful, since you will need to get a very high and precise pressure on a roadbike (around 100/110 psi), and very low and precise on a MTB (like 25/27/32).
Buying a pump expressing only BAR is not a good option. Answering to your question about too high pressure; consider that it is true that too high pressure will decrease over time, but it could increase if you live in a place where the climate/temperature is high during the day. It could cause the inner tube to explode, and damage your tyre, having to replace it. Buying a new tyre will cost much more than buying a track pump.
Buying a decent track pump costs only around 15 or 20 euros
If you are machinists you probably know about the “bouncing steel ball” material hardness test. The harder the material under test the higher the ball rebound towards its start altitude, always less than 100%. To your question .. The same principle operate in a reverse order to let it test how hard our wheel (ball) is instead of hardness of the concrete floor. Same principle applies to a basket ball. The catch is we need a standard for reference - ie our bike tire inflated to a target value. Measure how high (peak cm/inch) in first rebound and take it as a nominal record. We can, if we like, to also create a lower limit rebound record that will tell us if we need to inflate or not.