About 3 years of age is the earliest you can tell. It is something to do with brain development that is not complete until at least this age. As Tan32 says - it make take longer to tell. I agree with RayRay - most kids use both until about 3 years.

Someone was telling me this week about a study that was done where some left-handed people were tested and actually found to be right-handed through brain tests and it turned out that they were conditioned to be left-handed by their parents at an early age because they showed a preference for their left hand early.

HTH
TICKLISH