Valves are often characterised by two numbers - the mutual conductance, mentioned by radiozero, and the amount of (anode) current flow, mentioned by Peter N. The latter is controlled by, among other things, the cathode emission. Some simple valve testers don't measure mutual conductance, so their 'goodness' indicator has to be based on current i.e. emission. More sophisticated testers could, presumably, use mutual conductance as the measure of 'goodness' but whether they do or not I'm not sure.
On p4 of their KT66 data sheet
http://www.tubezone.net/pdf/kt66-mov.pdf GEC specify that the valve will have reached end of life when its mutual conductance has fallen below 5.5mA/V under the specified measurement conditions. Of course loss of emission will lead to loss of mutual conductance. In practice the two numbers are somewhat correlated.
Cheers,
GJ