True chain can be determined by comparing block height with the block explorer for PoS or PoW.
As a Sybil attack can fake a chain on either PoS or PoW and only comparing to a Block Explorer can verify the true chain for a syncing node.
That's true only for PoS, not PoW, PoW in this case is objective enough. It's the thing Vitalik Buterin calls "weak subjectivity".
TaPoS or Economic Clustering is however an interesting way to do that task in an automated way - you can see which chain your friends / your preferred services were using when they were transacting. The drawback: Everybody using multiple addresses for better privacy does not help. However, I consider it a meaningful extension for PoS.