There is a hole in those adapters that uncovers if there's a SVHS tape in it at the same spot
the SVHS hole is on a full-size SVHS cassette. It's possible that feature was missing on some cheaper adapters, but the JVC C-P7U, C-P6U (and rebadges), Panasonic VW-TCA7 (and rebadges) and I think also these ones
you get "new" on amazon
and elsewhere sold under a ton of different brand names (no idea who manufactures those) have the feature.
Detecting whether something is a SVHS recording is done electronically though, not sure how exactly but you can look up circuit diagrams if you are very curious.