Form each of your hands into a circle. Rotate your wrists, maybe even dislocate your shoulder, so your hands rotate like bike tires continuously. Position your head like it's the handlebars. Never break eye contact. Smile so wide you have to unhinge your jaw, showing your gaping maw.

If you are currently on a bike, point to yours, point to theirs, then thumbs up and smile. If you aren't on a bike, reconsider your life choices.

Say it! "Nice bike!". =)

I usually just yell out "I love your bike!"

If I had to go with a hand signal though, I'd opt for the chef's kiss gesture.

Get a small electronic "noise maker"—a device that lets you play a custom sound at high volume at the push of a button. Have it play a recording of someone saying "nice bike!".

Alternatively, just yell "nice bike!".

Point at them with your finger at a downward angle, towards their bike, then quickly transition to a thumbs-up/smile combo. Can't miss.

I once called out, "Wow! A Softride!" And the guy on said Softride smiled and waved. I wouldn't be surprised if he gets that a lot when he rides it.

How about the fonz but only one hand.

It's time to bring back the shocka