Learn Russian: Разве этот гвоздь не зацепил тебе каблук?

This is the dative of possession and detriment. Instead of saying the nail snagged 'your' heel, Russian phrasing says it snagged the heel 'to you'. Using the dative тебе emphasizes that this action happened to you and negatively affected you, a structure that is very common when talking about clothing and body parts.

What nuance does the word разве add to this question?

The particle разве shows surprise, doubt, or an expectation of a different answer. While a standard question just asks for facts, adding разве makes it mean 'Wait, didn't it?' or 'I really thought this nail caught your heel.'

Since гвоздь and каблук both look like they are in the nominative case, how do we know which one is the subject?

Word order and logic give it away. Standard Russian word order puts the subject гвоздь (nail) before the verb, and the direct object каблук (heel) after it. Since both are inanimate masculine nouns, their nominative and accusative forms are identical, but context makes it clear that the nail is doing the snagging.