Just in case you are tempted to participate in the following action....don't. Take a moment, remember this post and curb your impulse.
If a friend has been dating their significant other for over 2 years please, please, please don't rip her left hand out of her jacket pocket just so that you can see her bare ring finger. After seeing the finger, please don't shake you head and cluck your condescending tongue and make some remark about how her man is dropping the ball on the whole marriage proposal thing.
That is tacky, rude and will probably result in your relationship changing from friend to acquaintance.