Here's a pic of some "MAGA" fans.
https://www.independent.co.uk/news/world/americas/us-politics/trump-madison-square-garden-rally-photos-b2636532.html
I can't see the connection to having the IQ of a turnip, but perhaps it takes one to know one.
Maybe a better example would be: if they are not protesting at universities or elsewhere, (you know like yelling "from the river to the sea" or wearing a keffyieh), then perhaps we can be fairly certain that "they are MAGA folk."