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ckn, observation in wild field may not be very difficult to differentiate between Common Snipe and Pintail Snipe.  However, without examining carefully the tail feathers of a snipe, we could hardly differentiate Pintail Snipe and Swinhoe's Snipe.  The diagnostic difference is just the width of several tail feathers.

So you can say the one you saw might be Swinhoe's or Pintail snipe but the confirmation to be the former cannot be drawn.