Bamboo (kabob) skewers in the media. Gently twist the skewer into the media (for clay pots, as near to the center as you can get it; for plastic pots, approximately mid-way between the center and the edge of the pot, but anywhere away from the edge works). To check, remove the skewer and touch it to your cheek or upper lip. Even if it seems dry, but feels cool to your skin, there is moisture in the media. For Phals, allow media to be very nearly dry (skewer only feels cool, not damp), or entirely dry (but don't let Phals
remain dry. Replace the skewer back into the same spot you removed it from until next time.
A freshly sharpened pencil can also be used - gently twist into the media approximately half way, wait a few seconds, and remove it. The newly exposed wood will darken from moisture in the media.
I also like to put a small overturned net pot (or other small pot) in the center of the pot, or styrofoam in the center of the root mass - no soggy media at the center, when the rest of the media is dried out. Makes for happy roots!