Learn French: Est-ce que tu vois une empreinte sur la porte ?

You would replace une empreinte with the indefinite pronoun quelque chose (something) from this lesson. The sentence becomes Est-ce que tu vois quelque chose sur la porte ?

Doesn't empreinte mean "footprint"? How do we know it means "fingerprint" here?

The word une empreinte refers to any mark left by physical pressure. Context usually tells you what kind of print it is! On a door, it is most likely a fingerprint. To be highly specific, a fingerprint is une empreinte digitale and a footprint is une empreinte de pas.

Why is it tu vois instead of tu voit?

The verb voir (to see) is irregular. For the tu (informal you) form, it always ends in an s: tu vois. The form ending in a t is used for il, elle, or on (he, she, or one): il voit.

Can I ask this question without using Est-ce que?

Yes! In everyday spoken French, it is very common to just use the normal sentence word order and raise your pitch at the end: Tu vois une empreinte sur la porte ? Using Est-ce que just makes it clearly a question from the very first word.