Can you tell if two people are a couple just by observing them, if you don't know them?

[RideThroughTheRain]

If there is no open PDA like kissing/hugging/hand-holding, can you tell if two people are a couple or just friends? Is it easier to tell for heterosexual couples or for gay/lesbian couples, or are they all hard to tell? Is there a certain body language you associate with being a couple? (The way they stand, walk, look at each other?)

Scenario: You see two people from your neighborhood meeting regularly to eat, and they often go on walks together and visit places together. They seem close, often laughing and complimenting each other, but they aren't siblings. Would you assume they are a couple? Does it make a difference whether they are two guys, two girls, or a guy and a girl?

[kataloso]

I think it's usually pretty easy to pick up on the vibe if a guy & girl are *together* vs. just being friends or brother-sister or whatever else. And I guess I think in a healthy relationship it should have an obvious spark like that.

[FourStar]

Sometimes but somepeople are just touchy friends and it makes it hard to tell