how can you tell if they are contact prints?
This isn't a particularly important factor. As stated before
, since you can see the (badly) cut negative edges, they look very much like contact prints. It is also suggested by the crooked images. An enlarger rig would help prevent crooked prints and the image could be sized so that the negative edges don't show. If the image sizes match one of the standard negative image sizes of the time, they are almost certainly a contact print.