Start with structure
Compare the closed jaw to the rear of the eye, then record how deeply the spiny and soft sections of the dorsal fin are divided. These relationships are less sensitive to changing light than body color.
For largemouth and smallmouth bass, add the side pattern: a broken horizontal band is useful for largemouth, while vertical bars more often support smallmouth. Juveniles and hybrids still require care.
Use place as evidence
Clear rocky current supports a different shortlist than a warm vegetated backwater. Habitat never proves an identity, but it can reveal when a visual answer is ecologically unlikely.
Check current state or provincial records because both species have been introduced beyond parts of their native ranges.