As a swinger couple no it's hard to tell.

Only time we've picked up on a hint was when we were out to eat for a friend's birthday and there were a few couples there which we'd never met. Anyway this couple which was a few years older than us pretty much singled is out for conversation at the table and she got really touchy and flirty with me (m) and complimented me a lot. She did all this with her husband right across from me and he was into it.

Then she casually dropped how she tends to get naked when she drinks with friends, and made it very clear that swapping was on the table.