Here are a couple of articles on the techniques. It is not 100% but counts on the following:
FIFA uses wrist MRI for assessment of skeletal age to determining degrees of fusion of the distal radius growth plate.[14,15] In this method, the degree of fusion of the distal radius was graded independently into six classes in which grade I means completely infused and grade VI means fused growth plate. They concluded that complete fusion (grade VI) occurs at the age of 17-18 years.
Original article with MRI technique: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2465138/
Alternate lower cost method using ultrasonography: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4166061/