How to know if a stock is being held short

Redditors successfully attack hedge funds because they know that GME is being held short by them. But how do they know this? Where do they get this information?

According to older posts, you can check the number of short interest on NASDAQ’s site. However, GME is not listed on NASDAQ, so we can’t check like this. So where do redditors get the required information to beat hedge funds?