Practice.
HOW TO REGOGNISE MATING PATTERNS
A few years back I made some articles on mating patterns (since then, the viewer was changed and messed up the formatting a bit, but it's still readable):
http://blog.chess.com/batgirl/mating-patterns-i
http://blog.chess.com/batgirl/mating-patterns-part-2
http://blog.chess.com/batgirl/mating-patterns-part-3
http://blog.chess.com/batgirl/how-do-i-mate-thee-let-me-count-the-ways
http://blog.chess.com/batgirl/how-do-i-mate-thee-let-me-count-the-ways---part-2
Please share your ideas on what GM's do to easily recognise a mating pattern..