All I can say is look around online for your local antifascists. Be careful though, there are a lot of fakes. Make sure to look at the corresponding Antifa Checker for facebook and twitter. General lefty orgs like the DSA and the IWW will also be happy to help with stuff like this if you contact them
If none of that works out, get some friends together and become the antifascists you want to see in the world. Find proof in any way you can, then design a flyer with the receipts and hang them around town to raise awareness. Here’s an example that was put up in my city recently:
Get proof first though. Nobody’s gonna get up in arms for “this guy seems a little fashy”
It’s Going Down’s “Forming An Antifa Group: A Manual” may also be of interest
As for how to get proof:
Maybe you’re lucky and you can find the dude’s name in the Iron March leak. If so, you’d have all the heinous shit he’s posted to quote too. If not though, look up his name on social media (facebook, twitter, insta, etc) and see if you can find enough racist comments or dogwhistles to build a solid case