My friend works in a retail clothing store and she can immediately tell the difference between true customers and those with ill intentions. The first things she looks for are:
1. Whether or not the “customer” starts noting the location of security cameras and store staff. Regular customers don’t care about that
2. If the customer moves away from staff if they approach (whether subtly or not)
3. If the customer is overly defensive or won’t look at them in the eyes
And a bunch more I can’t remember right now.
Usually it’s not a single clue but a combination of all the “tells”
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