My fingers and eyes are as efficient as any water meter...
Its fairly simple. Pretty much already as said you need to water til there is some run off in soil. The amount of run off is really up to you and it pretty unimportant until you get to flushing.
I inspect my pots on the surface and through the drainage holes. The drainage holes are actually better indicators of when to water. The surface of the soil can get dry fairly quickly with there being ample water still remaining below.
A 'dry-wet' cycle is important but it isn't really what it seems. Plants gain most of their mass from interaction from the air, not water.
The term used in cultivation of fungus is called field capacity. You want your medium to be wet to the point where it will barely release any water, a drip or two, when squeezed firmly.
Also your plant will always tell you when it needs to be watered if you pay close attention.
I notice that my plants tend to have their saw edges up closer to watering time. I think this is due to slight ph changes in the media that occur through drying and concentration of solutes. Also it is a natural reaction from plants to v their leaves slightly before wilting in an attempt to conserve moisture. I can generally tell about 24 hours before the plant needs to be watered just by looking at the plant's body language.