Fistulosum will be the tallest, reaching 10 feet or so; the other 2 usually max out at about 6 feet. It has narrower leaves than the other 2. The lower portion of the stem should be hollow. The stem is tinged dark purple and is glaucous (with a grayish white waxy coating).
Purpureum will have bigger leaves than Maculatum. Its stems are only purple at the nodes and are not typically glaucous. It is the most common Joe Pye Weed in Missouri.
Maculatum has a flat-topped inflorescence with more florets than the other 2, which have a dome-shaped inflorescence. It is uncommon in Missouri.