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.
Have you listened to a single thing anyone had said in this thread?
Producing blocks under PoS has zero cost, therefore any desired chain height can be reached by the fake chain, making it impossible to objectively differentiate between fake and canon chains.