Parents shouldn't choose careers for their children but I believe there is a constructive role they can play. Who better than your parents to evaluate you critically but lovingly, but more importantly for the purpose of your question, critically? Who is in a better position than one's parents (assuming a normal, reasonably healthy family, of course) to advise one about where her strengths and weaknesses lie (remember, we can never see ourselves the way others see us), and in this way begin to provide us with guidance (only guidance).
Therefore I think careful observation of one's children from toddlerhood (I just invented the word 'toddlerhood') is absolutely vital! What kind of child is your toddler?
Is he quiet or gregarious? Is he detail-oriented and literal-minded or does he describe things in a more general, metaphorical, figurative way? And so on and so forth.
What kind of things does he like to do? Is he the child who can rig up surprising musical instruments and weave fantastic stories of wonder? Does he pay a great deal of attention to building blocks? And perhaps he shows an absolute fondness for numbers?
I'm just suggesting that from the moment children reach toddlerhood (I just invented the word 'toddlerhood'), parents should be on the look out for and recording those cues.