you know, I was thinking...just saying the soil is too dry or too wet doesn't say much and I am guilty of that too....
however, the best way to know when your plant needs water is to let it start to sag a little, then water it...it may take half a day or half a week before watering...
the shot glass you said you were growing in does not drain....you have to have drain holes in the container to let it drain...remember, plants are air breathers even through their roots so you want the soil moist but not sopping wet...
after you decide on the appropriate (for your case) container, just feel the weight of the container and it's contents including the plant before watering and after watering....this will let you know by a simple feel whether your plants need/want water/food....