You could use the sight, that'd pretty much tell you exactly what they were. But I'm not picturing a great life expectancy for someone who uses the sight to view a valkyrie in their full glory as they charge at them with an axe. Though I would expect them to need clean underwear if they did survive.
Lore roll would clue you in to them being more than human, and probably only extensive assessment would reveal their actual nature - it takes Harry a little while to work out what he thinks she is (other than some sort of nordic spell-caster).
If you wanted to tone down the catch rebate you could require it to be wood grown in Norway - or in the vicinity of a viking settlement - which does give you the possibility of a small amount US grown hawthorn - or at the site of a great battle. In any case it's not difficult to get hold of, just time consuming, which may have the same effect. And if you wanted to make the catch more difficult to achieve still you could require it to be from a particular grove of trees - or that it be an actual weapon rather than just an object made from hawthorn wood.
