Not if I'm looking at the block times or PoS participation rates. The attacker can feed me their own fork while preventing me from seeing the actual highest-difficulty chain, but their fork will be suspiciously weak, so I'll know not to trust it.

> Block creation have high variance. Over short periods of time, say a few hours no blocks being announced is fairly common. Over long periods of time how do you know the mining power has not decreased or increased at a slow rate than your person prediction.

Yeah; with Bitcoin you would need to wait several hours to get a good estimate of hash power. With more granular blockchains like Ethereum you could get a good estimate more quickly.