Eupatorium perfoliatum Common Boneset.
If you cannot tell at a glance, smell the plants. Eupatorium flowers have a pleasant fragrance. Rabbit Tobacco has a resinous smell, flowers and all. You might find it unpleasant; I like the herbal aroma of it.
Color is important. Eupatorium flowers are a bright white, Rabbit Tobacco is more cream colored.
The flowers are shaped differently.
Rabbit Tobacco clumps are usually more slender with denser flower heads.
Leaves are thick up and down the stem. By this time of year, the bottom leaves
usually begin drying into a silver color backed by brown.
Eupatorium has those lance-shaped leaves in a more open arrangement and
the flower head looks more like an airy bouquet.
Botanists give certain characteristics that we won't go into there, like sessile leaves and such. Do note that Eupatorium leaves are more lance shaped, while Rabbit Tobacco has many very slender leaves.
Clumps of Eupatorium among grasses and other plants.
Rabbit Tobacco among grass and dead stems of Erigeron and a yellow wildflower.
Don't let the exposure of this pic fool you, it made the Rabbit Tobacco look whiter.
That is your wildflower lesson for today. Test tomorrow.