Even after years of study, I only recognize three basic types: long dangly ones, tangled bushy ones, and tangled bushy ones with lots of apothecia. :)

The characters to pay close attention to in Usnea are:

1) Is it pendant (“dangly”) or bushy? (See U. ceratina for dangly, U. lapponica for bushy, for example.)
2) Is there any red or orange coloration on the outside? (See U. pensylvanica, for example.)
3) Is there red or pink coloration inside? (See U. mutabilis and U. endochrysea, for example.)
4) Are there soredia or isidia? (U. hirta has abundant “isidia”, U. pensylvanica has more typical isidia, U. lapponica has just soredia and no isidia, U. strigosa and U. trichodea have neither.)
5) Do the soredia form tiny spots smaller than half the width of the branch, or large spots greater than half the branch? (But be careful to note small soralia which coallesce into a large mass or irregular soredia toward the branch tips, as in U. cornuta. U. pensylvanica is a good example of small soralia, U. lapponica has large soralia.)
6) Are the soralia raised or stipitate, are they flush with the surface, or are the concave and eroded? (Eroded soralia can leave a flap of cortical tissue around the margin, such as in U. lapponica. See U. ceratina for stipitate soralia, and U. pensylvanica for flush soralia.)
7) Are the branches, esp. the main branches, goose-bumpy with papillae? (Often variable for any given species, but it’s usually conspicuous in U. ceratina, for example.)
8) Is the base of the thallus jet-black? (This can be ambiguous. U. subfusca is a pretty safe example, though.)
9) Is the medulla loose and webby or dense and solid, and is the central axis narrow or very thick? (Loose medulla generally means the secondary branches are constricted like sausages where they attach to the main branches. The typical example is U. cornuta.)
10) Are there conspicuous cracks encircling the branches, making it look like it’s made of a string of bones? (See U. trichodea.)
11) Are the branches round in cross-section or angular or pitted or ridged? (U. cavernosa is pitted. U. hirta is typically angular. If you find one that is “winged” like the branches of a winged elm, be very excited, this species – U. angulata – is very rare these days!)
12) Are there white-topped warts? (There is a subtle variation called a fibercle which has a tiny dot at the tip formed by a secondary branch breaking off leaving a scar where the central axis used to be. The best example of tubercles is U. ceratina.)

Of course, different species grow on rock and trees. And spot tests on the medulla are extremely helpful for verification, especially when we’re still learning what all these 12 characters really mean: lye and bleach are all you need for Usnea.