Here are a few things to look for in foliose lichens:
1) how closely appressed to the substrate (doesn’t get much less appressed than this one!)
2) does it have rhizines (little root-like things holding it to the substrate)
3) does it have cilia (rhizines sprouting from the margins instead of lower surface)
4) what color is it above and below, noting especially any white mottling on the surface, either in the form of dots or network of ridges or lines
5) does it have apothecia (cup or disc-shaped fruiting bodies)
6) does it have soredia or isidia (fine to coarse granules, or minute finger-like or globular outgrowths that break off easily in your hands, respectively)
7) size of the lobes (it’s very hard to decide what a lobe is in many, but after you’ve seen lots it will be clear that some are much broader than others, e.g. compare how much broader most Parmeliaceae are than Physciaceae)
If you have access to bleach and lye, a few spot tests can make all the difference in determining certain genera.
See the discussion under Lichen sp. for more information.
(PS. This one looks somewhat like a common species, Platismatia glauca, we have in North America. If it is, I would expect to see no rhizines and a distinctive black and white-splotched underside, with coarse granules forming at the margins and spreading inward in age. It should be pretty uniformly gray on top and somewhat dull but not dusty.)
