I generally soak the soil with water until the soil is moist. I check the moistness of the soil each day and if it's still moist to the touch, I leave the seedlings be.

Generally when moist then I leave the seedlings be; when the soil is dry to the touch then I water again. Normally when I water the seedlings the soil remains moist for 3-4 days if it's cool and cloudy out. If it's really hot out then generally by the late afternoon (between 2:00-4:00 p.m.) the soil needs to be watered again.

Am I underwatering? What are signs of underwatering? I really don't know how often (how many times per week, per day, etc.) to water. Does discolored leaves on my radish seedlings indicate they're ill? What should I do to help them? I really don't know.

I am wondering if they're deficient in some type of nutrient and am considering giving them fertilizer. However
they're only a few days old and I'm wondering if it's too soon to fertilize them. In general how old should seedlings be when you start to apply fertilizer to them (1 week, 2 weeks, etc.).

If the cause of discoloration is because of hot weather, there is really nothing I can do about that, except move them to partial shade to cool them down, which is what I've done.