How to tell doneness of a boiled egg besides time

Try an egg timer that measures temperature.

Here's an example (that I don't necessarily endorse): Egg Perfect Timer

The idea is, the device supposedly absorbs heat similarly to an egg, and gives a visual signal of the level of heat absorption. If the water is colder than typical, because you're at a higher altitude, the time it takes to heat the eggs and egg timer will lengthen, but the amount of heat the eggs and egg timer need remains consistent.

I had one very similar (perhaps that same brand) when I was much younger. I remember it being hard to read, but I think it was accurate.

My parents never used a timer. They lifted the egg out of the water and observed how quickly the water evaporated from it. (This would be a measure of the egg's temperature.)

If you don't mind using a dozen eggs to find out, try lifting one up, timing how long it takes to dry, then cracking it open, do another, etc etc until you know what you're looking for. At first you might actually need to time this drying process but I am sure it's possible to learn "hm, yes, this egg needs some more time" vs "yup, just how I like it" within a few weeks of normal egg consumption.

(Found an answer (first one here) from a uk person stating that 10 seconds is the time for soft boiled - personally I would want to test to discover the time, but ok, and the person mentions living at altitude also.)