The all inclusives use wrist bands - as do the ships for children under 12 - not a lot of gawkers in the under 12 age group.....

I think that statistically cruise folks are more likely to be new to Club O and show more obvious curiosity than those on a land vacation - and there are standard beach towels if you know the ships - so I think you can guess substantially better than random chance as to whether a person is a cruiser or not - but that doesn't tell you anything about the individual person other than whether they are there on a port of call or not, anyway.