The wiki page for "Dvergr mage" could fit the OP's description; it's not exactly clear whether the resistance to fire and frost are "across the board", or if (as would make sense) the fire mage is resistant to fire, and the frost mage is resistant to frost.
https://valheim.fandom.com/wiki/Dvergr_mage

I suppose the easiest way to tell would be to hit them with something you suspect they're not resistant to (ie, hit the fire mage with cold damage), and see what color the damage numbers are; grey = resistant, white = normal, yellow = weakness.
https://valheim.fandom.com/wiki/Damage#Resistances_and_Weaknesses_(Tenacity)