There’s a really cool idea from an outfit called Botanicalls, which is selling a little kit that senses soil moisture in a flower pot or planter, and then goes out on the internet to tell the owner (via a tweet) when watering is needed.  It’s available here for about $100.

This device is somewhat expensive (but fun!), when all you really need to do is insert a finger in the soil to get an idea of when to water, as nicely explained here at Petunia

The oldest and most proven method of judging a plant’s need for water is the simple touch test.  With your index finger, feel the soil down to about one inch deep.  If the soil feels dry, add water; if the soil is moist, do not water and check the plant again in the next day or two.

Or, if you want to go electronic, but not on the web, there are less expensive devices, such as this one, or this one, or the cool Dr. Frog sensor for a few dollars.