What Sida says; When someone in your party uses Limit Break, you'll be tied to the animation of cheer (you can still move and do what ever you're doing when it happens) and you'll hear the sound that it is currently going on. If you're not sure who's using it the moment it happens, look at your party list and it's the slower casting action with a special name. Braver, Bladedance, Big Shot, Desperado, Skyshard and Starstorm being the most common as they're Limit 1 and 2 from dps.