I think this answer is not very pleasant and in fact it spoils a bit my fun
with free software.
There is a strong need for mechanism that provides more transparency and
control for the users of precompiled free software, because very few users
can build really everything from source.
Although I use GNU/Linux for many years and have a rather technical
background, I just don't know how to build a whole Distro like Trisquel from
source.
If someone who knows it could write a tutorial, this would be a first step
towards the transparency I mentioned.
Of course it makes more sense to trust the distro developers of Trisquel than
a company like microsoft or apple, but just taking packages someone has
compiled for me misses the point of free software and breaks the strongest
argument of control.