ceestyle said it... u really can't overwater seeds, especially if u have them in a perlite-strong, peat mix with water-retaining properties(like the perlite), but it's when the plants develop a root system that you can do damage by over-watering... They will literally suffocate because of too much water and become unable to transfer any nutrients, and possibly develop root rot... I use the "pick up the bucket" method described above, as I use this method, and know what it feels like after I've given them a good watering, and I know what it feels like after a day or two, when it needs water again... You can't truely tell if your plants need water, solely by sticking your finger in the top three or so inches of soil, as it can still be very water-soaked towards the bottom. Ed Rosenthal gives some good examples of installing a clandestine "water level meter" one can put in each container with a tube, bamboo stick, and a cork, that may be useful to you, as well...