If the bed is about 14 and a half inches wide, then it is a full size. I'm not very good at being able to see the differences between the 3/4 & full size machines, unless they are Singers. Sometimes I have to go to a spread sheet to double check.
Like the difference between a 66 & 99 can be seen where the stitch length screw is in a slightly different position on the pillar and sometimes you can tell by how far the decals from the arm go above the needlebar.
The difference between a 27 & 28 is the location of the hinge pins.
Janey - Neat people never make the exciting discoveries I do.