For a pine that's been collected in either fall or spring, for me the most reliable indicator is that it then goes on to make it through the following summer, autumn, and winter to produce healthy shoots the spring after that. So basically one full cycle of post-collection warm + cold seasons.
By then, if there were issues with re-establishing roots that somehow remained hidden or obscured by preexisting momentum (as mentioned by Shibui above), they'll have become very obvious. This is assuming the tree somehow makes it through the heat of summer without showing those signs first, but some may be blessed with exceptionally mild summers.
But in all cases of conifer collection I think it's better to simply wait until you have the kind of outstanding vigor and growth that could only be explained by drawing copious amounts of water from the soil. Regardless of whether there are signs of growth early in the recovery process, you're still ultimately waiting for this to do any work or significant movement (i.e like physical transport of location) of the tree.