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