Many people initially separate the Goldenrods by whether or not they have small compact flower heads, or widely branching flowers like these above. I don't start that way, but prefer to use habitat. Rather than open fields, we'll look at the Goldenrods that tend to occupy forest areas first.
Moving to drier habitats, this is a common species of poor soils. The Gray Goldenrod, Solidao nemoralis. Gray Goldenrod is one of the smaller species reaching only 1-3 feet.
Key features include the leaves and stem being covered with very fine hairs. This often gives the plant a grayish-blue look. Leaves are widest near the tip and taper down at the base. On most plants I've seen, the leaves also get smaller as you go up the plant. Gray also has a habit of sending out little leafy shoots between the main leaves. Some say they also recognize it by how the top of the flower head doesn't stay erect, but bends over slightly.
If it's Sweet Goldenrod Solidago odora, the lower leaves will have shriveled up and turned brown, or disappeared completely. Of course there is one easier way to tell. If you crush the leaves and get a pleasant anise or licorice odor, you have Sweet Goldenrod.
Well that's only half the species in the state. Perhaps I can find many of the others next year. Of course being in the field with an expert might help!