The tools for digital signing already exist and are used in niche industries but not really something everyday people are concerned about. It's more a social awareness problem and this will come with better tools, integrated with the apps and channels ordinary people use daily so that it is something that happens in the background.
In addition to digital signing, public distributed ledgers can establish provenance or a chain of ownership for signed content, so digital content that is altered and redistributed can be sourced back to its origin more accurately.
Ordinary people with the right tools built into their devices will be able to see very easily "Is it in the chain?" based on a simple enquiry to the ledger, and if the answer is no, then it's untrustworthy.
If the answer is yes, then they will ask "Does trace to the origin?" and if it does not, or the origin isn't signed by the alleged owner, then it is untrustworthy.
That's one possible way crypto can help defeat disinformation / fake news content. The underlying techniques exist but there is a lot of work to do to bind it to everyday use.