[Amps] How Do You Know When Filter Caps Begin to Fail?

[Pete Smith N4ZR]

I have an SB-220 that dates at least to 1978.  There is no external sign 
of any problems, and the amp has been heavily used in contests for at 
least 20 years.  It still delivers over 1300 watts on 80-20.  Is there 
going to be any warning of a capacitor failure?  Will there likely be 
any cascading damage to other components? Is it conventional wisdom that 
I should replace them as a precaution?

-- 
73, Pete N4ZR
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