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Sometimes you can tell just based on the description. (Don't ask for examples, because I can't remember any at the moment. But I definitely remember times I've realized an idea would work as soon as I heard it.)
What you can't predict (or at least I can't predict) is how big working turns out to be. Largely because that depends on the ability and drive of the founders, which is hard to judge when you meet them, let alone predict 5-10 years later.
Can you get customers (CAC)?
Will they be delighted (NPS)?
Will they stick with it (long term cohort retention)?
Only then can you answer viability.