There isn't a way to tell besides weapons, I usually assume beast types and things that aren't holding a weapon are strike type.
so this is all but assumption? hmm......
wouldn't it be something like this?
[Monster's Raw ATK + Elemental ATK (if any)] - [Your Damage Reduction+Element RESIST(if any)] = Monster's DMG Dealt.
Monster's raw damage meaning, neither strike nor slash nor pierce. Regardless of what weapon they use to carry, they just deal raw damage+element(if any). Weapons on them are just visual representation. This is what I always believed in.
Since this issue with slash,strike,pierce in PvE somewhat interests us, and as you've said you "assume" beast types fall under STRIKE, my guildie had to test if Hunter's Tiger would fall into one of this category but sadly, his tiger cant break a miniboss. Not slash/strike/pierce. So why would I bring this up with a summoned tiger? Well to show that "UNLESS STATED" monsters just do Raw DMG. Same with Mecha Jins, Falcons, and Behelders. Pets (dogs,cats,birds,dragons,kitsus,etc) however, can break since its attack attribute is defined.