You know great sex from the mediocre. The latter is a ho-hum experience that you simply endure. You pant, very slightly. You might think of Leno and who's on The Tonight Show. And when it's over, your partner collapses on your less-than-enthralled body and exclaims it's the best sex he's ever had.
Contrarily, great sex is loud, exhilarating, sheet-grasping, sweating, panting and an endless cacophony of pants, moans, squirms, and sometimes religious screams that keeps the bedroom rocking and wakes up the neighbors.