It's impossible to prove, by design, because any fighting force is going to have a mixed composition. Those libs are gonna claim anything that portrays the West/the West's friends in a bad light as "propaganda". The general composition of pro-Euromaidan and anti-Euromaidan should give you an idea of who's on which side. Sure, the Nazi composition might be overstated, but there sure are a lot of Nazi groups on their side, as well as various groups described as "ultranationalist" "national conservative", etc. Certainly gets the old noggin' joggin' even if they arent "Nazis".

There's also the fact that the US is currently prosecuting American far-right people who got caught committing war crimes in Donbas. One link.