My rule is if I get a flat and can't find what caused it, I'm going to get another flat. I keep checking the tire, the tube, the rim, rim strap, spokes...everything. 9 times out of 10 I find the cause. That 1 time out of ten I don't, and if I slap another tube on the wheel, I invariably get another flat on that tire in short order.

I've been known to use a corner of my bike jersey to check the inside of a tire for burrs, glass bits, steel-belted radial tire wire, etc. Works like a champ if one can't find an obvious cause for the flat.