It depends on what the circuit is and how the tube is being used. Miniature duo triodes ( 12AX7 etc.) have two triodes in one glass envelope. Each section can perform differently: transconductance, noise, and so forth. Power tubes are matched up to provide balanced drive to the primary of the output transformer in power amplifier. Measuring your small signal tubes will require you have a mutual transconductance tube tester, characteristic curve tracer, or some other device to do this. Such test equipment runs into the hundreds of dollars for a good used tube tester and much higher for the curve tracer. George Kaye makes a small signal tube tester that is currently manufactured and allows you to plug ear phones into it and listen for microphonics. Neat! Unless you want to invest in tube test gear you may want to obtain your tubes from someone you trust who can test them for you and provide you with tubes with balanced sections. Under most preamp conditions I'm not sure obsessing over whether your tubes are precisely matched is either necessary or critical.
