From the heat and current, aging occurs on gain and leakage.
By measuring the plate leakage current and mu that affects voltage gain they could test the loss of output swing and thus loss of function of an inverter Triode.
How long does it take for you to see if one tube is going or gone?
The 1st ENIAC has over 7000 tubes so if it replaced 3 tubes every day then that is an MTBF = operational hours ÷ # of failures = 24 * 7000/3 = 56,000 h which is the lifespan of a modern high-quality tri-phosphor 48" fluorescent tube running 24/7.
By the end of its operation in 1956, ENIAC contained 18,000 hi-rel vacuum tubes, 7,200 crystal diodes, 1,500 relays, 70,000 resistors, 10,000 capacitors, and approximately 5,000,000 hand-soldered joints. It weighed more than 30 short tons (27 t), was roughly 8 ft × 3 ft × 100 ft (2 m × 1 m × 30 m) in size, occupied 1,800 sq ft (170 m2) and consumed 150 kW of electricity.
In the 70's I had my fave Bogan stereo tube amp and I would go to the drug store every year or two to test the tubes and perhaps replace 1 if it was glowing blue plasma or weak.