If you want to be really old fashioned, you can ask
¿Cúyo té es?
Where cúyo/a/os/as is an interrogative that needs to agree with the possessed item and is directly equivalent to English whose?.
But in modern day Spanish, that is not used (and so you shouldn't either, unless you're a native speaker, in which case you should to bring it back to life ;-) ), and instead you use
¿De quién(es) es el té?
The plural form is used when you expect a plural answer, similar to English's whose vs who all’s.
When making general statements of possession, you will most often use the unstressed possessive adjectives in front of the possessed noun:
- mi(s): my
- nuestro/a/os/as: our
- tu(s): your (inf.)
- vuestro/a/os/as: y'all's (inf.)
- su(s): his, her, its, your (frml.), their, y'all's (frml.)
Because su(s) has many different potential meanings, it's common to clarify by using [def. art.] [noun] de [pron.] For example, instead of saying su perro when we mean it to belong to a guy, we can say el perro de él. Or instead of sus libros when referring to a group of girls, we can say los libros de ellas. While vastly less common, you can do this same structure with first and second person pronouns. While vos/nosotros/as/vosotros/as don't require any special work, yo takes the special form mí and tú takes the special form ti: mi libro = el libro de mí, tu camiseta = la camiseta de ti). In most situations, you'll need to add on mismo/a for it to be considered grammatical (el libro de mí mismo), and the de nosotros is generally only heard in the Americas.
You can also use the stressed possessive adjectives, which also allow for indefiniteness (the unstressed ones shown above act on their noun as if a definite article were there and in fact in old Spanish it was common). These stressed ones go after the verb and will agree in gender and number and can be nominalized (in which case they correspond to the English possessive pronouns that end in -s:
- mío/a/os/as: my/mine
- nuestro/a/os/as: our/ours
- tuyo/a/os/as: you/yours (inf.)
- vuestro/a/os/as: y'all's
- suyo/a/os/as: his, her/hers, its, you/yours (frml.), their/theirs, y'alls (frml.)
These are quite common when answering questions regarding possession (but notice, sometimes clarification is needed using the de [pron.] format:
¿De quién es el libro?
¿De quiénes son las manzanas?
— Pues, son suyas.
— Digo, de esas niñas.