Determining Sufficient Moisture in a Flat and Watering Techniques
Flats that are sufficiently moist will show some condensation inside their lids.
If you're not seeing condensation it could be that you have too many vents for air transpiration or too many drainage holes, or possibly, because the flats are in a mostly shaded location they are not warming much inside and so no steamy vapor can rise to adhere to the lid's inner surface. You can tape over a few slits...see if that alters the condensation effect.
Moist soil is dark looking.... like the color of a cooked very-fudgy brownie. Dry soil looks dry.... it's far lighter in color and that color reminds me of a package of dry chocolate cake mix.
To add more water to a flat remove it's cover and gently dribble in some cold water....don't slosh it onto the soil surface as that could dislodge the seeds or seedlings. Give the soil a good soaking, any excess water will seep out through the drainage slits. Replace the lids promptly and securely.
To add more water to a flat made from a bottle simply tilt the bottle at a slight angle, then gently and slowly dribble some water into the bottle, the water will run down the inside of the bottle wall and won't splash directly onto the soil surface.