Well for one, define 'adequately watered'. Secondly, a reliable continuous sensor will be a tdr sensor, the cheapest of which start around usd 200 - and then you have to do a bunch of hacking yourself still. It's not so easy - there's a reason real indoor growers don't use pots with soil. Look into using other mediums, or maybe use a cyclic flooding system, but both require adapting the environment the plants are in, they're not so nice to look at with the tubing and overflows etc.