Begin with the whole frame
The most useful clue is often not a single fitting but the relationship between hull, superstructure, mast and working deck. Start by recording what is plainly visible, then compare it with dated official imagery or catalog descriptions. A likely resemblance is a lead, not a final identification.
Separate image evidence from caption evidence
A caption can carry a date, place or ship name, but captions travel and are sometimes copied. Keep the original caption, the repository record and the visual observation in separate notes. When they agree, confidence rises. When they do not, preserve the disagreement instead of smoothing it away.
Use a repeatable worksheet
Record the image source, catalog number, visible markings, estimated period, likely vessel type and unresolved questions. The same short worksheet works for a family scan, a museum image or a page preserved by the Internet Archive.