Steve Branam replies: The clue is in the name. The dovetail joint is named for its resemblance to the tail of a dove, the way it spreads out from the point where it’s rooted to the dove’s body. The side of the joint with that same shape is the tail side. It’s narrow where it’s rooted to the end of the board, and spreads out equally to the edge. Typically, either the tails and pins are spaced equally, or the tails are larger than the pins.
From the point of view of the grain orientation, the tails angle diagonally along the face grain, then are squared off straight across the end grain. The pins are straight along the face grain, then angled across the end grain.
Bob Oswald replies: The dovetail side looks like a dovetail. The other side has parallel edges. Very noticable on the through dovetail. On the half blind, the holes are the pins. The part that goes into the holes is the dovetail. It looks more like a dovetail than the holes.