a good cheap way to test tubes

Simply put; a woman.

My girlfriend noticed my rig was sounding harsh in the highs. I thought they were a bit bright, and something else was missing but I couldn't place it. Finally I changed out the tubes in my amp and it was all better. The highs were perfect, bass improved and the mid range sweet sound was back.

I'll admit, may not actually be cheaper than a tube tester, but they do have their perks.