How can I tell the difference between a rabbit and a cat?
In Germany we have an old (joking) saying that roughly translates to "head off, tail off - bunny", so your question is legitimate. But first thing's first: There is no health risk1 involved if you ate the latest shipment of "meowling rabbit". (To cat lovers everywhere: This is no endorsement, I have a much loved and pampered cat, too!)

The most obvious differences are


head (look at the teeth!), 
tail (short vs. long), and 
feet (cat claws are quite distinctive). 


But I suspect these would be removed prior to sale, so we'd have to dig a bit deeper into anatomy:


Shoulder blades: 
Cats' shoulder blades are rounded on top, especially in the front, making the shoulder blade almost semicircular with the ridge making for two differently shaped parts, rabbits' shoulder blades are triangular with the ridge somewhat parallel to the front side. 
Femur (thigh bone):
A cat's femora are straight, whereas a rabbit's is more or less bent, especially pronounced in older animals. Same applies to a lesser extent to radius and ulna (forearm), but they might have been removed.
Ribs:
Cats' ribs are "rounder" than rabbits' ribs according to this source. I can't say by how much, but @aitchnyu's comment below suggests the difference is obvious enough to play a significant role in the novel/movie Papillon.
Pelvis:
The same source as above states that the foramen obturatum (= the loop-like opening near/below the hip joint) is oval in rabbits and round in cats. I couldn't find a good picture to show this, though.


Here's a rabbit:


Source: www.onlineveterinaryanatomy.net

And a cat:





1 other than the general risk every meat from questionable sources brings