" A tube in its last breath"

My amps and preamps are all tubes, but I do not have the faintest idea to know which of the tubes are failing. Are there signs and symptoms I need to be aware of short of buying myself a tube tester?( which can get very expensive)
I need your help!
Usually tubes die slowly. Some symptoms to look out for are:

- less high frequency extension
- less dynamics
- tube bias will change noticeably (if a tube suddenly goes bad)

Hope this helps
(1) increases in idle noise floor & (2) microphonics
are two degradation mechanisms that are normally first detected by ear & are not even measurable with most common tube testers