There are things called soil moisture meters on a sort of spike you can buy - the spike goes into the soil in the pot. But to be honest, that just means you still have to look at each pot with its moisture meter to read the level - if you have to do that, you might as well not bother and simply pick up the pot to see how light or heavy it is - if its light it probably needs watering. Or just touch the top of the potting soil with your finger sideways on - if its dry to the touch, then water. You want to water when it feels like that, but before the compost has shrunk from the sides of the pot.

Moisture meters are intended to be inserted into the soil, read, and immediately removed and wiped clean. DON'T leave them in the soil, since that corrodes the sensor at the bottom of the spike, and they give false readings.