The best way to tell whether your succulent is being over or underwatered is by the appearance of the leaves. An underwatered plant will have wrinkly, shriveled up leaves whereas an overwatered plant will have soft, mushy, almost translucent leaves.
Beside above, how do you fix Overwatered succulents?
Dig the succulent out of the soil and remove excess soil stuck to the roots, cut off any brown/black roots as these are rotten already. Leave the plant on a mesh or any kind of strainer till the roots have air dried from anywhere two to three days. When the roots are dry completely, plant them back in the pot.