There are several approaches one might take:
1. Thoroughly measuring all characteristics to assure that they all fall within the expected range of the presumed taxon.
2. Finding the "model number on the label ", a characteristic found in only one taxon.
This might require a lens (e.g. profuse glandular hairs on Dryopteris intermedia, or the spherical glands on Thelypteris simulata).
3. Keying it out.
4. Feeling the gestalt of the plant, and knowing its name.
5. Making a very good guess while driving by on the interstate. Reading habitat and gesture.