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What I often do is simply walk up to a free check-in desk and drop by bag on the belt to weigh it.
Not that in some airports, if the desk is closed, the scale will be switched off, so you often need a position which is manned but with no one using it, so you’ll have to make a big smile to the person at the desk. It may be more or less difficult to find such a desk depending on the airport and time of the day.