DYC's? Yes, Damn Yellow Composites. Let's tell it like it is. This is one of those difficult groups to get to species. I'm going to attempt to separate many of those I have seen in this area, both wild and in plantings. Rather than deal with a genus all at once, I will use morphological characters to 'try' and keep them apart. Since this is an extremely large subject, I will post in several parts.
A) Does the plant have very showy rays? (Rays look like flower petals)
B) Are the rays extremely reduced or totally absent? Let's deal with these first.
A) Majority of leaves simple
B) Majority of leaves compound or divided.
Rays reduced or absent
A) Phyllaries averaging 6-8...Stick-tight, Bidens frondosa
B) Phyllaries averaging 10-12...Stick-tight, Bidens vulgata
Now on to the showier ones.
A) Main stem of plant with distinct WINGS...we'll do these first
B) Main stem of plant without wings...4
1a) Leaves opposite... Crowns-beard, Verbesina helianthoides
1b) Leaves alternate...2
Continuing with alternate
2a) Flower heads compact, gumball-like, rays 3-lobed...3
2b) Flower heads loose, spreading, rays unlobed, drooping below flower head...Wingstem, Verbesina alternifolia
3a) Flower heads light colored, yellow-green...Common Sneezeweed, Helenium autumnale
3b) Flower heads dark colored, purple-brown...Purple-headed Sneezeweed, Helenium flexuosum
Plants with most leaves alternate. What's this 'most' bit. Sorry, some can be both.
4a) Leaves primarily basal, large and broad, up to two feet in length...5
4b) Leaves smaller, more well distributed along the stem...6
5a) Leaves almost as wide as long, elephant ear like...Prairie Dock, Silphium terebinthinaceum
5b) Leaves similar in size but highly divided, looking fern-like...Compass Plant, Silphium laciniatum
6a) Leaves simple...7
6b) Leaves compound or divided...8
7a) Leaves elongate, covered in soft velvety hairs...Black-eyed Susan, Rudbeckia hirta
7b) Leaves shorter, broader, either smooth or with a slight rough touch...Brown-eyed Susan, Rudbeckia triloba
8a) Leaflets divided into narrow segments, flower head gray/black, rays growing downward, drooping...Gray-headed Coneflower, Ratibida pinnata
8b) Leaflets broadly divided, uppermost simple, flower head yellow/green, rays spreading outward or only slightly drooping...Green-headed Coneflower, Rudbeckia laciniata
Those were the easy ones. The impossible opposite branched ones are coming. I only say that because none of them fit easily into a "simple" key, and part two may end up as just species descriptions.