One thing about being aware early of just what trajectory the kid is on is if you want to plan for life accomodations to adapt to his needs. Like moving to an area with great gifted programs (and highly gifted programs,) or getting into a situation where home schooling is feasible. Or even researching a pre-school context that won't stifle.

I found the Ruf Estimates to be interesting for at least a thumbnail sketch:
http://www.talentigniter.com/ruf-estimates