Please have at least one nearby friend who can see how you're doing at any time.
Here are some ideas in case you choose to go undercover. I've not tested all of them.
Don't smile. Smiling makes them suspicious. Shouldn't be too hard not to smile amongst fascists anyway. As long as there's nothing specific to fake-laugh at, try to mimic a Moai statue's facial expression.
Roam some White Nationalist forums beforehand, so you'll have something to talk about just in case. It also allows you to piggyback on the "conspiracy" mindset you often find amongst them. You could, for example, claim that forum X is secretly run by far-left Socialists or something.
Maybe even contact that specific group beforehand and "join" them in order to gain their trust.
You could sneak your own flyers amongst theirs, with instructions on how to "join an underground Nationalist network" or "make your phone hacker-proof". Did you know that an Android phone starts formatting itself as soon as you type *2767*3855# where you normally type numbers? No warning messages. No confirmation. The factory reset starts as soon as you type the last character.
Hand out free cookies that taste awful.
Handing out
white roses is probably safe, because one needs to know history to get the reference.
Bad smell has already been suggested. It's a subtle way to lower their chances of recruiting new members. Sneakily putting a stinking substance on their propaganda material could be effective. Butyric acid smells really bad.
Put on some annoying music, pretending to be dead serious about it.
Needs some considerable skill, effort, and/or money, but
https://gizmodo.com/5829551/free-t-shirts-given-to-neo-nazis-contained-a-secret-message-after-being-washed
