How do I know I can trust the head of the blockchain
Your node doesn't know for sure. Actually, nobody ever knows for sure that the current chain-tip will be in the chain with the most work, it just becomes increasingly likely as more time passes and additional blocks build on-top.

Once a sufficient number of blocks have been built on top of the observed block, it will be sufficiently unlikely that the observed block will end up in a stale chain-tip, and you can trust it. 

Users make different judgment calls about the number of blocks they wait for: some don't wait for another block and accept at first confirmation, some wait three blocks, some require six. The extreme is likely to be the 100 block waiting period the network requires before mining rewards may be spent.

However, until proven wrong, the node will assume that he has the newest information: after all finding a block is an unlikely event, and thus will respond with information about his block when queried for the blockchain-tip.