You cannot. Not with any software you didn't write or very carefully review. It's always about trust.
In the case of your example, I think gorhill is a prime example of a trustworthy author. He has a very good track record, never betrayed the users, explains his thought process and behaved consistently in the users (my) interest in the past. uBlock Origin is the one extension I trust the most, even more than say the Multi Container extension from Mozilla.
My worry is not the author or the extension; it's someone being able to push a new release of the extension (e.g. signing keys compromised, some vulnerability somewhere, etc), and it being automatically approved by Firefox without manual review.
In the case of your example, I think gorhill is a prime example of a trustworthy author. He has a very good track record, never betrayed the users, explains his thought process and behaved consistently in the users (my) interest in the past. uBlock Origin is the one extension I trust the most, even more than say the Multi Container extension from Mozilla.