in the northeast, you can always go with the keys at the Native Plant Trust: https://gobotany.nativeplanttrust.org/dkey/symphyotrichum/ or https://gobotany.nativeplanttrust.org/dkey/symphyotrichum/#all.
for the entire US, you can use the Flora of North America key: http://floranorthamerica.org/Symphyotrichum, along with the species maps from BONAP: http://bonap.net/NAPA/TaxonMaps/Genus/County/Symphyotrichum to narrow down species in the area.