I had a couple phals planted in moss, from the vendor, in dark green
pots. These almost never needed watering, as they held too much water
and led to root rot. I saved both by moving them to bark, in
transparent pots.

If your pot is transparent, then you can tell when the roots inside
look dry and you can water then. If it isn't transparent, then having a
similar orchid in a transparent pot (along the lines as the previous
poster but with a plant in the pot) can serve as a guide. So now you
have an excuse to buy another orchid!

But if you want to be really sure, you can remove a few bark chips and
peer into the bark below with a flashlight. New bark is light when dry
and dark when wet. If the bark inside is becoming light again, then
it's time to water. After you've done this for a few times, it'll
determine a regular schedule that you'll only need to adjust if the
temp & humidity (in the growing area) change radically.

-Munir