Technically, the storage center is situated below the navel, around two inches.
You don't have any way of knowing immediately if it's working, you just assume it is. There is no 'downside' to this, because the centers are designed to store energy and spill it over upwards, so that if it's doing what it's supposed to, your natural abilities should improve, whatever they are. In other words, you don't 'overdo' it, because once a storage center is full, the next one gets the extra energy, and the center itself eventually becomes able to store more energy.
Kind of like a gentle workout, you eventually get more strength or flexibility or whatever the desired result is without immediate pain, and the more you do it, the more you can do it.