First test should be a blacklight. It's not necessarily conclusive if it doesn't fluoresce, as others have shown several modern papers that don't, but if it does, it's a dead giveaway that it's a repro/fake.
If it passes that test, then start looking at paper type, thickness, etc.
Also keep in mind that if it's valuable, someone has tried to fake it. Some fakes are more prevalent than others, but just because you haven't seen one does not mean they're not out there.
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