gently seperate the plant and pot a little and if the soil stays together and is filled with roots its time to pot on (rootball). now there is probably a much better way of doing things but this works for me. Just dont be heavy handed and its probs best if ur wrist is fuckoed to let someone else do it. be real careful not to damage plant when ur doing this. If you place ur hand over the top of the pot with the stem prodruding from inbetween 2 of ur fingers you can turn it upsidedown and lift the pot a bit to see what the rootball is like without damaging the plant. seen horties do this with other plants all the time....
its just common sense man.....