[Silicon Sam]

There will be a stamping on the engine block with the CC displacement. But the quickest way to tell is the carb tops are angled outward (and shorter), and the intake manifolds are shorter. Front timing belt covers are smooth, not ribbed for your GL1100 pleasure. And lack of mechanical fuel pump on the GL1200.

These are not the only differences, just the most obvious.

Got a link to the ad?

You'll find that a good majority of online sellers have no clue what they have. I see it all the time in the local CL.