How do I know I can trust the head of the blockchain I'm trying to understand how the mining process works. Specifically, my understanding is the local miner works on blocks and add them to their local blockchain if they succeed. It may turn out that this isn't the longest chain of blocks (because there are other miners working in parallel) and when that longest chain arrives at the miner, the miner will discard its local (shorter) subchain in favor of the new one.

My question is if the local node (who's also a miner) is asked for information from the block chain before that longest chain arrives, how would it know that these blocks may still change in the future and shouldn't necessarily be trusted?

I apologize if the whole question seems confused. It's only a reflection of my own confusion on the topic.
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.