Originally posted by Subsonic:

If you have the hood, just put it on--do you now see fire resist among your buffs? If not, then you need the whole set.

That would be a more reliable method if the bonuses consistently actually showed up in the buff list.
The root helmet doesn't have the poison resist show up anywhere for me, but I know it's working. Same with the Wolf Armor frost resist. It's not showing in my buff list, but I know it's working, because I don't freeze in the mountains.