If you don’t know how to read code, then you pretty much have to trust them, and all other open-source software out there. The good thing with FOSS is that there’s probably someone who cares about it enough to read it and audit it, although there can also be a chance that no knowledgeable person cares about the code so no one ends up actually knowing what it’s doing.
I don’t know how to read code, so I pretty much have to trust all of the FOSS that I use. Although open-source is usually more trustworthy than proprietary counterparts (read: PRISM)
by the same logic, they won’t know what you do inside Tails, nor when you boot it up