So investigate them. If they are. Then shoot them. Enough times to make sure.
Just look at the US. You can’t let that shit percolate. Whatever guardrails you’ve placed your faith in. They aren’t enough. Kill them all. Cut them out. Burn them out. Root and stem. Or they will destroy you in time. They’ll find the cracks and use a crowbar on them.
That sounds great but I don’t believe a Western style liberal democracy is able to do that due to how enmeshed Nazis are in their organisations on every level, from regional to international, and how central fascism is to the project of saving capitalism in the face of crises.
It’s the same with saving the planet and any other project demanding radical change - we’ve learned time and time again capitalism can offer nothing but empty promises, to actually achieve anything you need to first establish the dictatorship of the proletariat.
As an added bonus, DOTP is able to re-educate and de-radicalise communities targeted by fascist indoctrination by placing their needs first, raising their standards of living, and allowing them to flourish and develop a healthy communal spirit, so only the worst offenders, leaders and ideologues, need to be executed.