"Hol látod az autót? = "where do you see the car?"
"Hol látsz autót?" = "where do you see a car?"
Hungarian sometimes likes to drop the indefinite article, especially when we're asking for where things are. "Hol látsz egy autót?" doesn't feel right at all. So when you're asking for where one exact thing is, you have to leave that egy. Hope this helps.
This is virtually the same question as "how do you know what's the direct object of a verb". To some extent, you just have to learn it but it's more or less similar to English, except English hardly marks accusative (for direct objects) and dative (for indirect objects). "To see" is a transitive verb for example - "a car" is the target of the action. "To give" has both a direct and an indirect object typically - to give someone (indirect) something (direct). The direct object is the affected thing of the action, the indirect object is the receiver. Most basic verbs follow similar patterns in Hungarian, too - expect both direct objects and indirect objects receive some kind of ending (-t and -nak/-nek respectively).