Simon Toomer in Trees for the Small Garden says "Large plants left too long in a small container may be pot-bound, a condition that may prevent root spread once planted out. A mass of roots projecting from the drainage holes at the bottom of a pot is an indication of this."
One visual clue indicating that the tree has been in a pot too long is a dense cover of weeds and moss over the potting compost along with a mass of entangling roots at the surface. If the container looks too confining for the roots, it probably is and has effected the tree's vigor.
The photo of the maple above shows several visual clues that the tree is pot-bound.
