1. 'Inspect' an item in a Shop or your Inventory to see it's values. A lot of Armor has flags like Ablative or Plated.
2. Your Codex has an 'Item Mechanics' entry that tells you what those flags mean. (Database->Items->Technology)
3. The game generally isn't difficult enough to make min-maxing Resistances vs most enemies necessary.
Shields are almost always weak to Electric, strong(ish) vs Kinetic. There are exceptions.
Armor resistances don't matter until you take Health damage.
Tough's bonus Kinetic Resistance applies to Health and Shields.
you could look up the specific shield or armor on the wiki, but in general shields are weak to electricity, except some very rare exceptions. Armor has no unified weakness, every armor is different