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