Finger/thumb lengths, angles, and meeting places can vary from hand to hand. But no matter what your hand is shaped like, the base knuckle of the index finger is still the fulcrum that drives the lever of the racquet. All the geometry of the swing is based upon the position of that knuckle, so it is the only truly accurate measure of which grip is being used.
On a true western, that knuckle is directly underneath the grip, driving it straight upward for max topspin potential. Try it, and see for yourself if any other part of the hand is instrumental in the driving force behind the racquet. Try it in a variety of grips, really FEEL how that part of the hand is the ONLY part that drives the racquet, and you'll soon see why the position of that knuckle is the most logical way to determine grip.
Looking at the thumb position and other such visual tells can be thrown off if a player prefers smaller grips (and the grasping fingers thus go a little further around), or larger (the opposite). But even with off-sized grips, the base knuckle still has to be in position to be the driving force. It is the one constant.