Without beating to death the difference between polymyositis and dermatomyositis, suffice it to say polymyositis is the disease without skin manifestations, whereas DM does have skin changes.
The skin changes associated with DM are as follows:
1. Gottron's Papules (see above). Violaceous papulosquamous eruptions over the MCPJs and IPJs.
2. Heliotrope rash. The classic violaceous rash on the eyelids is textbook, but it may be subtle at times.
3. Shawl Sign. Diffuse, flat erythematous rash over back, neck, upper chest (often in a V shape)
4. Facial erythema
5. Periungual telangiectases
6. Flagellate truncal erythema
Not skin, but still part of the diagnostic picture:
7. Proximal muscle weakness (ask them to stand up out of a chair without using their hands)
8. Muscle tenderness, particularly the proximal thighs.
Just like with Beau's lines, the best place to find DM periungual telangiectases is the fourth finger.
Although this is a discussion about adult DM, there is a juvenile variant which also has cutaneous calcifications.