Quoting this Leviton article:
- MOV life is largely dictated by thermal stress
- Exposure to surges cause heat-ups and cool-downs of the device, as well as (depending on the nature of the surge) incremental damages such as electrical puncture and thermal cracking
- These incremental damages lead to non-uniform heating when additional stresses are applied, which leads to thermal runaway and device failure
A surge protector that is connected to the mains should be considered 'on the clock' - once you get past the manufacturer's recommended replacement time period, you're on your own. Unless you have access to a surge generator you won't really be able to test if the MOV is still 'good', and since you could be in a 'one-and-done' situation (the next surge gets clamped but takes the MOV out), it's not really worth testing it.
You could (theoretically) replace a questionable MOV with a comparably-rated one if you just want protection and don't care about dealing with manufacturer's warranties. They're available from a variety of suppliers - energy-handling and voltage clamping level are the key spec items to look out for.
The safest thing to assume is that if the surge protector is older than the manufacturer's equipment guarantee, assume the MOV isn't in there at all (i.e. there's no protection).