One way u can usually tell is if the roots of the plants are brown or mushy. This might also be a sign of root rot. Which would mean seeing wilting and yellow leaves.

If you need to know the moisture level they have readers which you buy at home depot for around $5 - $7 dollars that will help you out. Most of them read from a 1 to 10 range with 1 being bone dry and 10 being to wet. Another way you can tell is by inserting a pencil. If the pencil is damp when you pull it out then u know it's watered pretty good.

hope it helps :P

edit:
Do the pencil suggestion or your finger for the 1st inch