How do I tell if an enemy has overwatch?

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Sometimes an enemy will attack one of my units with overwatch if I briefly peek out of cover. Other times I can stand directly in front of an enemy for as long as I want without issue.
How can I tell if an enemy is prepared to counterattack my units with overwatch?
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What Your Overwatch Main Says About You
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Francesco Ungaro, Eren Li, Ketut Subiyanto, Ketut Subiyanto