How do you ask someone what there hobby is in french?

The real Word is "violon d'Ingres" but it sounds a bit weird and old. It might still be used if it's your main hobby and you have a real passion for it.

Another name for it is "passe-temps", but it has a slight negative connotation of "time waster".

Many young people will just use the English term "hobby" or "hobbies", or will for instance not mention the term at all in their resume, directly mentioning their activities under a "Personal" section, for instance "je pratique le Rugby" or "je joue de la guitare" (I play rugby or guitar).