It's the rear-view mirror and steering wheel that usually give it away; that usually tell you you're sitting in a car that was primarily designed to have a roof, but has one no longer.
Normally it happens within a couple of minutes of getting in the car. Meet a complex set of bumps and the steering wheel emits a little shimmy, the image in the rear view mirror goes blurry and, all of a sudden, you're aware that all in the body structure is not as it was intended to be.
For this reason, among others, convertible versions of coupes are not generally my favourite cars. When they're outwardly supposed to be sports cars, even more so.
And so to this week's Autocar road test, of the Audi R8 Spyder. A very sporting former-coupe whose top has been lopped. But not with all the usual results.
Seldom, if ever, have I driven a car whose roof removal has resulted in such a small perceived loss of body rigidity.
It helps of course that the R8 is a two-seater with high sides and a small roof aperture. A four-seat coupe with no roof has no chance of retaining 80 per cent of its body stiffness like the R8 Spyder does. Lord only knows what Porsche's mooted four-door Panamera cabriolet would be like.