It can be nearly impossible to distinguish a normal PAL 1200 mirror from the break-resistant type, unless, of course, you know exactly what to look for.
When producing the normal type of mirror, the elliptic shape is first cut from a sheet of silver coated mirror (front coated). The cutting leaves a very sharp edge, which is then removed in an after treating process.
The process of producing the break-resistant mirror is a bit more complicated. In this process, the non-coated glass is first cut into shape, then after treated to remove the sharp edges, tempered to make it break-resistant, and finally silver coated. This process leaves the treated edge with a silver coating, where the normal type is blank (polished).
Therefore, to see if the mirror is tempered, see if the treated edge is silver coated.