yes, you can tell before they reach puberty. and, from my other post in skills and drills:
okay...here goes.
skeletal muscles consist of bundles of individual muscle fibers called myocytes. each myocyte contains many myofibrils which are hairlike strands of proteins called actin and myosin which hold on to one another and pull. this is what shortens a muscle and causes a muscle contraction. without the process you have no 'twitch' at all.
there are 2 types generally speaking. slow twitch type 1 and fast twitch type 2 muscle fibers. and these muscle fibers can be further categorized as type 2a and type 2b. and each fiber type has its own unique ability to contract in certain ways. muscles have a genetically determined combination of fast and slow fibers both. the average would be 50/50 slow and fast in most muscles that are used when we move.
slow twitch muscles are super efficient at using oxygen to create more 'fuel' known as ATP. ATP is adenosine triphosphate which is a major source of energy for cellular reactions that occur in muscle tissue. kinda like those "rods" that sit in a nuclear reactor. the more ATP that is produced means the longer a person can go with extended muscle contractions over a long period of time before fatigue sets in. this describes your typical marathon runner or marathon cyclist.
fast twitch muscles are more efficient producing short bursts of strength and speed because fast twitch fibers use anaerobic metabolism to produce fuel. and this metabolic process will find the muscle fatigue more quickly. this describes the sprinter, a top tiered gymnast and Michael Jordan. Larry Bird and Michael Jordan both had huge vertical jumps. and even though Larry Bird was taller, Michael Jordan could get to a measurement peak faster than Bird. And because Jordan was so strong he could jump higher also. but thru their entire career Bird was more minutes on the court and Jordan took more breaks. fast twitch muscle fibers will generally create the same amount of force in a muscle contraction as that of slow muscle fiber but get their name due to the ability to fire more rapidly.
type 2a fibers are a combination of both. but type 2b fibers are the gladiator fibers to end all. these fibers excel at producing the fast and powerful bursts of unbridled speed that you see in certain athletes in certain sports that are more likely the result of preordained genetic make up. if you put Bird and Jordan on the 100 meter dash who do you think will win? but what if you put them both in a 5K? Jordan wins the sprint and Bird wins the marathon...both hands down. it's biology.
finally, you ask how this would be determined. medically speaking there are tests and measurement comparisons and muscle biopsies for just about everything. but for us? you'll know it when you see it. the double back on floor that looks labored and slow yet still has a deductionless outcome vs. the opposite of labored and slow with a deductionless outcome but wins by .025. they just look different...understand? or the famous 2 step slam dunk 2 feet above the rim performed by Jordan and others biologically similar compared with the 4 step push thru the defense running in sand like labored movement of Bird to accomplish the same task only 1 foot above the rim and whose play action took 3 times longer on the clock.
ahhhhh...the wonder of human biology.