Originally Posted by alan s

Not too hard to tell.

Just out for a ride: often in pairs, have no bag, are smiling and weaving, generally getting in the way, slow down before passing others
Touring: four panniers and other stuff strapped to the back, moving slowly and often in small groups
Heading to/from training ride: kit and no other gear, riding at a relaxed pace, but way faster than me, repeatedly spitting or blowing snot rockets
Bike commuters: moving fast with a determined look on their face, making near miss passes, bag on back or bike