How do you know when a gem levels?

With so many skills that you can equip how does the game let you know when one of the skills you've socketed levels? do you glow or something? Just seems like it could be hard to keep track of whats about to level.

You see a progress bar on the gem itself as well as a message in the chat window when it levels(this info gleaned from livestreams, it may be inaccurate or incomplete).

I'm hoping they get around to adding some sort of a particle effect and a sound queue whenever a skill levels. As of now (at least from stream watching) it seems like it would be hard to know exactly when they level. As tool mentioned though there is a progress bar you can monitor whenever you highlight the gem.

As they said, each gem has a exp number. you can see here.

When they level up, you get a message in the chat that says Cleave has reached level 6!

A leprechaun will jump out of your screen to let you know each time.

Your Gem has leveled. Congrats for opening your eyes!