Some testers, like the Amplitrex, do test at full power and do give one an idea of how much life is left in the tube. This tester, if attached to a computer, will curve trace the tube. Someone who owns dozens of modern and vintage testers told me that he likes the E-Tracer which is bought from Taiwan as a kit; it too curve traces and delivers very close readings to the Amplitrex. Another supposedly good tester/tracer that comes as a kit is the Sofia tester.
The Amplitrex is extremely easy to use. You don’t need to look up test parameters to adjust it; just find the tube type in its dropdown menu and it will do all the settings automatically and then produce a detailed readout including the expected readings so you can compare actual to expected readings. The primary downside is that it is expensive.
A local shop uses a TV7 tester as its main unit. It is good for finding bad tubes and weak ones, but, it tends to be a bit generous in its rating—it will give strong readings for tubes that the Amplitrex says is not that strong. This shop also uses a modern, very compact tester called the Orange, but they mainly use it for small signal tubes.