Well perhaps one way is you could have another LLM take a look at the data you are submitting and have it predict p(useful|not useful) , and create an incentive for users to generate authentic data
This must be quite an insult to the inventor of fully decentralised AI. In 2012 Prof. Jelasity invented gradient mining with SGD, https://arxiv.org/pdf/1109.1396 Works reasonably well, you probably also need a ledger like Trustchain for accountability and Sybil protection. Consensus is too costly. This problem is unsolved and actively worked on academia for decades.
you can randomly ask sometimes 4 different nodes to calculate the same gradient, and see who is cheeting.