You can use the console command "pos" to reveal your characters's coordinates in the game world. (To use the console, you need to add the "-console" command line parameter before you launch the game on Steam, and then you have to enter dev mode by typing "devcommands" after opening the console. The console opens when you press F5 if you did the command line parameter correctly.) I think the Y coordinate is the elevation, but I'm not sure about the units. It could be meters, it could be something else too I guess.

The next part is going to be a major spoiler. If you care about world gen spoilers stop reading right here.

All of the possible mountains in the tenth world are basically known at this point. The highest mountains even have names. This is because the map of Valheim is basically a random location within a slightly bigger map which is the same every time. Each run of the game takes place somewhere within this map (Image credit to community member Mooey on the official Discord): https://cdn.discordapp.com/attachments/441573352197324801/909151366495240252/SPOILER_Complete_Map.png

If you have explored a good chunk of your map you can probably identify the mountain you're looking at, and you can see how it ranks compared to the other peaks.