These are often patchily distributed or restricted to a small part of the thallus. Usually it will be a green alga, but not always. It's best to check until you have experience. Very dark lichens often have cyanobacteria, but also some lighter ones. Check a cross-section under the dissecting scope.
Cyanobacteria look blackish.
This is an occult art. Many lobes swell or fork at the tips. Measure behind the swollen or forked area. Some thalli have lobes of different sizes. When this happens, you have to guess what the author of the key has in mind.