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The book cover theory of genre fiction, as articulated by Debra Doyle, aka me:

Short (classic) form:

If it’s got a rocket ship on it, it’s science fiction
If it’s got a unicorn on it, it’s fantasy.
Once a book has a rocket ship on its cover, the only way to change it back into fantasy is by the addition of the Holy Grail

The inevitable smart-ass in the back of the room: But what if it’s the Holy Grail in the shape of a rocket ship?
Me: Then you’ve got the Hugo Award.

Special bonus side-cues:

If the cover features a person in heavy-duty powered space armor, it’s military sf.
If the cover features a person in a fancy uniform/dress coat with gold braid and similar decorations, it’s space opera.

A zeppelin on the cover means alternate history.
A female person in corset and bustle, or a gentleman in a top hat, in the presence of either gears or zeppelins, means steampunk.

And finally:
Any otherwise mundane cover can be made into a fantasy cover by the addition of random sparkles.