I just wandered into this forum after seeing a few youtube vids about this very subject:
https://www.youtube.com/result.... What I do is knock off enough of the nursery pot soil to get a good look at the roots, then make sure at least some of them are hanging downward instead of growing up and around the root ball. If its rootbound you'll know it - the roots will all be wound around the rootball - nothin' hangin' loose. Sometimes you have to get tough and really pull the roots away from that ball, or even cut right into the root ball to loosen them up. Those videos will show you what to do.
