This is the easiest tell. The authentic zip top uses a custom YKK zipper where the pull tab is shaped like a tiny audio wave. Bootlegs use standard hardware.
With high demand comes counterfeiters. If you are searching for the keyword "fred again ten days zip top" on resale sites like Depop, eBay, or Grailed, you need to know what you are looking for. Here is the authentication guide: fred again ten days zip top
If using eBay or Grailed, search the exact keyword "fred again ten days zip top" and filter by "Newly Listed." Beware of sellers in China shipping from the US; stick to sellers with "Concert Proof" (a photo of the item on the actual tour date floor). This is the easiest tell