There are basically three species of Vernonia in Indiana. I say basically, because there are a couple of very rare species and, the species that are here are variable and tend to interbreed. All of this makes it somewhat confusing to identify exactly which species you’re looking at. My tendency is just to call it Ironweed and be done with it.
However, hear are some pointers.
Giant Ironweed, 20-30 flowers per flowerhead, smooth leaves
Missouri Ironweed, 35-55 flowers per flowerhead, hairy leaves
Prairie Ironweed, 20-30 flowers per flowerhead, smooth leaves with dark dots on the underside
Keep in mind that these characteristics can be variable, and the plants interbreed. Hmmm.