Here's a little trick. Carry a 9V battery in your pocket at swap meets or junk yards. Whether you are considering a used orig T3 or an NOS one, touch the battery contacts to the 2 prongs on the bulb before you buy it. IT's enough juice to light the filament if it's still under vacuum or toast it if it's leaked and full of air.
The metal contacts that go through the glass are not sealed with what's called a glass-to-metal seal. That would take a metal known as Kovar (which has the same thermal expansion rate as 7052 Corning glass). They were sealed with a ceramic potting compound which has a tendancy to become porous with age, and let's in air. The bulb may look perfect, but the compound may have dried up and allowed air to seep by.
It's not much extra load to carry in your pouch if your roaming the fleas and hope to find good T3s!
And whatever you do, don't test them again when you get home with a battery charger (way too much current). Even a good one will toast!
Given enough time and determination, good original ones can still be found. Better that than repro ones. Originals were dated with a stamp on the rear. Repros don't have that, like OSRV mirror glass. You get nothing on the backs; a dead give-away!
Verne