Or do you just do it cos in the grand scheme of things they’re not expensive.

The obvious visual tell is that there’s oil leaking out. No oil, no problem

…and no water getting in (milky looking oil bath).

When you do a lowers service you should be able to see what state they are in.

Surely it’s impossible to stop water getting into the oil of open bath forks in these conditions?

It is possible to stop water getting in. Make sure the wiper seals are in good condition, and don’t jet-wash near the stanchions.

I’d say best to play it safe and replace regularly, I change mine every 6months. If you can see signs like water getting in and oil coming out you’ll be starting to cause damage to other parts of the fork and I suspect your forks won’t be feeling their best by this point either.