There are several good youtube videos about fake pins.
Some highlights… coloring may be off, weight may be significantly lighter than a comparable oe pin, there may be misspelled words on the back, the waffleing pattern has an edge and or is incomplete, and if a hidden mickey pin the hm is not clearly defined.
I also have heard about noticeable brush strokes and dips in the enamel.
I am probably forgetting a bunch.
If you are trading in the park, just be aware of what you see many duplicates of, those are most likely scappers (or the new hm).