Most people don't have the equipment to test if tubes are matched. Sellers will compare the tube structure and the mutual conductance but people who really understand this will say that is just a fair guide.
According to Bob Putnak, at tubesound.com who is one of the gurus...
(4) the fourth school of thought is “total distortion” — meaning that output tubes are truly matched when the total distortion of your amp is lowest vis-a-vis another set of tubes.
Neither a tube tester, nor a curve tracer, measures an amplifier’s output distortion. For that, you need an automatic distortion meter and low-distortion signal source, and a lot of experience. The tubes are placed into the amp, biased correctly, and driven to max output before clipping. Distortion is measured. Tubes that do not pair well together will have higher residual distortion than a nicely matched pair. This method also allows you to measure the amp’s power output from each pairing of tubes.
In my opinion, this is the only real method to accomplish “matching”, although it is not strictly a “tube-matching” issue and therefore beyond the scope of what most end-users want to tackle. It is holistic health care for your amp. You are evaluating all aspects of the amp simultaneously and how they interrelate to each other. You are tweaking the entire amp (including the tubes) to achieve the cleanest output. You are tweaking the bias. You are tweaking the “AC balance” / “phase inverter”. You are swapping/mixing output tubes. You have objective scientific evidence (the distortion numbers) to prove how your amp is working. When you can’t get the amp any cleaner at the same power output level, you know that the amp is optimal within its design limitations. This requires a lot of experience as a technician and quality test gear.
Honestly, this is the “right” way in my opinion and anything else is a serious compromise. But since this would require everyone to become a technician, folks are looking for an “easier” or “simpler” way to match tubes.
You can read the entire text here.
http://tubesound.com/2010/10/27/tube-matching-with-a-tube-tester/