You can measure the loudness with a decibel meter. They're fairly inexpensive to acquire on Amazon. However, a quick trick is to hold the earbuds at arm's length with their music playing as usual. If you can still easily hear the music, then you know it's too loud for them.
Usually, there would be a warning that would pop on your screen when the volume is too loud. You can use that, in general, as the minimum level, then decrease the sound from that if you think it's still too much.