How do you say very happy in Spanish?

There are two ways to say this in Spanish -- the formal and informal way.

The Formal (Usted) is used with adults and people that you don't know well.

¿Está feliz?

The Informal (Tú) is used among friends and close family members.

¿Estás feliz?

Note the user of Estar instead of Ser for these translations. Because happiness is an emotion, you must use a conjegation of Estar instead of a conjegation of Ser.