First of all, you can jab your finger into the topsoil and feel if it's wet or not, that will give you an idea. You can also weight the pot by lifting it up slighly and if it's watered properly, it'll be heavy as a mudfugger
If you want to get scientific (and maybe it will ease your mind) you can get soil hydrometer, you stick it into your soil, keep it there, and it will show you how watered it is.
There is a low tech hydrometer of sorts out there that looks interesting, but I've yet to find 'em in a store or anyone that's actually tried them. Neat idea, though...little terracotta worm. Since it's that reddish clay stuff, it will absorb some moisture and change color to be more reddish when it's moist. So the idea is you can watch the color of the worm and supposedly tell when to water from that...
Wormie the water sensor:
