I have employed the toothpick/wooden dowel method that I have learned here. Stick one of them into the soil, all the way down to the bottom of the container, leave it for two minutes, then check to see if the dowel is dry. If it is, then its time to water lightly.

The top 1/2" or even the top 1" of the soil can be dry, but the rest of the container can still be holding moisture.

Exactly what Jif wrote :-)
Take the guesswork out of it with a toothpick.
For larger containers, I use a kabob skewer.

Josh

Swenson: Jif and Greenman are right. However, I would maybe rethink the egg carton method. Like you said, they can dry out pretty quick as there is very little soil that you can pack into an egg carton cell. If I were you, I would invest in a 36 or 72 cell seed starting kit. They are only about $5 and will likely make thing a bit easier on you as they hold more soil in each cell. You can also get a separate dome or some come with the dome lid which will hold in moisture while they are germinating. But you CANNOT use the dome lid in direct sunlight or you will fry your seeds/seedlings. You don't need them in light until after they sprout, then you can remove the lid and place them in the sun.
Bruce