The tack of a sail is the corner between the mast and the boom, for the mainsail. It will (on the main and other sails) display the sail maker's logo, which is probably the easiest way to identify which corner it is when it isn't rigged. The two other options are:
H - Battens used to maintain the form of the sail, especially in light winds
G -Â Luff, the front edge of the sail that begins to 'luff' (essentially flap) when the sails are too close to the wind.