There were metal detectors at the Museum of Natural History the last time I passed by (July), and I think they have them at Air & Space, too. They have "express lanes" for people who don't have bags or backpacks.
The smaller museums and galleries (like the Hirschhorn) inspect bags but don't have metal detectors.
The Capitol & Washington Monument have metal detectors as well; others, like the Lincoln Memorial, don't.
You can always call ahead to ask, though I can't imagine that flashlights would be a problem, in any case.