Mr.WakenBake is correct, the soil does need to dry, this not only helps fight root rot, but also allows air to get into the soil helping it to breath and encouraging the microbes that release nitrogen. Don't push it untill your plants are drooping though. The twist on this is that mj likes water as it does fantastically well if near a water table. Deep long drinks are the best as this also helps flush the soil like a good rain does naturally So all in all you start out with a good soaking, avoid the finger test,(though it does work), because there will be small spider roots looking for food and water and breaking them by accident just slows your grow, after your good soaking watch how the soil is drying, I do like the small probes as per tigerdude, and feeling the weight of the container is good untill you get a BIG container, then your just going to strain your milk lifting it each time. The small probe will allow you to get a read from several locations for a more accurate picture of how moist the soil really is. Great question, and the truth is you will pick up on your babies needs fast just naturally.
smoke in peace
KingKahuuna