Formal:
* Vous amusez-vous ? (This is the most direct and formal way to ask.)
Informal:
* Tu t'amuses ? (This is the informal way to ask, used with friends and family.)
* Ça te plaît ? (This literally means "Does it please you?" and is a more casual way to ask.)
* Tu passes un bon moment ? (This means "Are you having a good time?" and is another informal option.)
More colloquial:
* C'est cool, non ? (This means "It's cool, right?" and is a very casual way to ask.)
* Tu kiffes ? (This is slang for "Are you enjoying it?" and is very informal.)
The best way to choose the right phrase depends on who you are speaking to and the situation.