from what i've read its very difficult to tell a tortoises age without knowing its hatch date. others feel free to correct me.
the best method will probably be to have an expert estimate its age based on its size. of course the size can vary with its environmental conditions...thats why its an estimate.
Josh is correct without knowing the hatch date you will never know how old they are. There are so many different factors that cause torts to grow and then not grow. I just recently read a thread on another site that a guy did, it showed two torts the same age one was twice the size of the other one. It is just to hard.