Historians don't pick "good guys" or "bad guys" in fact, we're trained to purposefully avoid doing that.

And we have a lot of evidence from the German side of WWII, you can buy Mein Kampf in many bookstores (some country's don't allow it, but it's easy enough to find if you do). Our archives are also full of Nazi Germany plans, journals, manuscripts, etc. We really couldn't ask for more when it comes to WWII.

As for how to do we identify propaganda? Well, that's a huge part of what historians do. We don't just look at a source and say "well it says here that.." we need to take what the source is arguing, and investigate it. We need to compare it with other works, we need to compare it with other sciences and approaches. We can then analyse the data and produce an argument with that evidence. It's a process, but it's why there's so much training and credentials that historians need to acquire and why there's systems in place to make sure historians have followed the proper steps and didn't get caught up or tricked.