My belief of natural talent is this. There is a rate at which you acquire a skill, and if that rate is higher than average, you have a talent for whatever the skill is.
If it takes you 1 year to accomplish what takes most people 5 years, given the same level of effort and dedication, you have a talent for it.
Measuring that is unfortunately pretty difficult, but that is the basic concept I believe in.
You of course then have those whose passion/dedication exceeds that of the average person.
T = Talent
E = Effort expended per year
Y = Years spent pursuing
L = Skill Level
T * E * Y = L
A talented person with a an arbitrary value of 2 in T (1 being average), who spends .5 effort (1 being average) will gain skill at a rate equivalent of a normal person. They just appear to take less effort at doing so, and can acquire multiple skills at the same time.