In the textbook Text-book of meat hygiene: with special consideration to ante-mortem and post-mortem inspection of food-producing animals (Edelmann & Eichorn, 1908), pages 64-65 concern determining the difference between a cat and rabbit:
The following differences in the skeleton are especially to be mentioned: The lateral processes of the lumbar vertebrae which are directed forward terminate in the rabbit (Fig. 34) in two extensions, of which one is directed forward the other backward; in the cat they terminate in a point. The body of the first three lumbar vertebrae in the rabbit contains thorn-shaped ventral processes (Fig. 34 a). The ribs of the rabbit are flat and broad; those of the cat are rounded. On the scapula of the rabbit the acromion turns around posteriorly and terminates in a long point, which is directed backward (Fig. 37)). Radius and ulna are completely separated in the cat (Fig. 35); in the Leporides they are united (Fig. 36). On the humerus of the cat is an elongated fissure over the median condyle of the distal end (Fig. 39). The femur of the rabbit (Fig. 41) contains below the trochanter major, a specially strong smaller trochanter, which is absent in the cat. Tibia and fibula are complete in the cat (Fig. 43); in the rabbit (Fig. 44) they are only separated in the upper half.
The whoe carcass of the cat can be immediately recognized by its head, penis bone, and the tail, and for these reasons, if offered for sale, these parts are always removed on the slaughtered animal. Rabbits generally have the shot wounds, but these are naturally absent in the slaughtered domesticated rabbits. The meat of the cat is paler than rabbit meat; the fat of the cat appears whitish in contrast to rabbit fat, which is honey-yellow.
As it says, it has some explicit figures which you can go and see, showing the different bones of the one next to the corresponding bone in the other, so that you can easily see the differences.
If you don't speak anatomy, let me translate. See also the image below.
This is a 1908 book that tells food inspectors how to tell when someone is selling cats as hares/rabbits. They say that, for a true rabbit:
- the fat will be faintly yellowish rather than white;
- the forward-spikes of the vertebrae in the spine should sometimes have a subtle Y shape near their ends, ending in a little "prong";
- the ribs will be noticeably flat-and-broad rather than rounded;
- there will be a little spike on the shoulder-blade near the forelimb;
- when you look at either cat or rabbit, both the fore- and hind-limbs, like our arms and legs, have an upper-arm/thigh with one big bone (humerus/femur) and a lower-arm/calf with two smaller bones (radius+ulna/fibia+tibia): in the rabbit both of these lower-two-bones are fused close together on both fore- and hind-limbs, whereas in the cat they're noticeably separate bones;
- there should be a little hole in the upper-arm bone (humerus) right above the elbow; and
- on the side of the "thigh" bone where there's a "ball" connecting to the hips, you should also see beneath the biggest "lump" across from the "ball" a lesser "lump" coming after it.
How will you see these? To look at the bones you can probably just stew one of the animals pretty hard until the meat is "just falling off the bone," then carefully pick the meat off.
Make sure before you use any of these criteria to conclude catness that you have seen the difference on between two images of skeletons. Sometimes the difference is rather subtle! For example in the images that Stephie has posted, you can very easily see the "shoulder-blade spike" (labeled #33) on the rabbit, but not on the cat, so this is a clear diagnostic criterion: if I didn't see it then I'd strongly suspect "cat". Similarly the fused/unfused bones are pretty decisive, as long as you don't accidentally remove the incredibly-thin tibia of the cat and then think "oh, I just see one bone, it must be fused." Be cautious until you compare to both.
I've taken an image from Flickr and updated it with some highlights of what this 1908 manual is suggesting you look for: