Headphones

Turn the volume up to a level at which it's satisfyingly loud. Note down the volume number.

Turn the volume down from there until it's just a bit too quiet to be listenable. Note down the volume number.

Now set the volume roughly half way between these two numbers.

That's a pretty good way of getting a volume that'll almost certainly be safe to listen to (in fact it's likely to be a bit lower than you could really safely listen, but it's great if you're wanting to be extra cautious)

Then you should have no worries about hearing loss or anything.

Obviously listening too loud for extended periods of time is bad, so avoid that, but listening at moderate volumes is completely fine.