I think that if your line is noisy, then the surge protector is bad. If it's quiet then the surge protector is likely bad. That would be my guess based on the two instances I experienced.
lol WHUT? Either way it's bad?
Okay I get what you meant, but let's back up a minute...
How to test whether a surge protector still works depends on how it's designed. Each discrete component has its own test method.
Unless you're a rocket surgeon (aka electronics guru) who has a bench supply capable of generating high voltage, and reverse engineer the surge protector to determine what the clamp voltage should be, then instead you inspect the components.
Again it depends on how it is designed, but to start with, if you see spark gaps with burnt bridges, or MOVs cracked apart or pinholes in them, they are out of service. Similarly any blackened component is suspect. I am over-generalizing now, some things like TVS or GDT, are probably fine if they look okay. Shunting a large surge then failing to provide further protection, tends to be destructive visually.