If I were DMing, I'm not sure I would allow masterwork tools for UMD at all (in part because they amount to a "+2 to almost everything you do" for artificers, who are already a very powerful class.)

If I did allow them, one possible form would be a pair of gloves made out of some magically-sensitive material; as you try to activate the item, you can feel the changing magic fluxes in your fingertips, and use that as feedback to whether you're doing it right. A pair of goggles with magically-sensitive lenses would work along similar lines. In both cases, they don't let you detect magic at will (unless they're enchanted to do that as well) because they detect changes in the item's aura as you try to activate it, not the unchanging aura of a magic item just sitting there.