In fact, the prudent validator can’t know for sure.
But the design of Casper slashing (the individual being punished more if many fail at same time) anyway works in favor of those who avoid doing what everybody else does. It incentivizes decentralization in a very generic way, including e.g. trying to not use the same software and distribution channels everybody else is relying upon.
As a result of this multi-dimensional decentralization (the term diversity seems to fit), it should become much harder for an attacker to get in control of a large chunk of the network.
This could also mean that for non-techies it’s safer to ask a friend to run a validator node for them instead of using some one-click solution which requires no understanding of the system and which many others may be using too.
I think that’s better for network resilience, thus am skeptical about attempts which try to make running a validator node very easy.
We will see how it plays out.