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How do you know when Micro Snitch spies on you? I couldn't find the source code anywhere.Source code isn't really necessary. Nor sufficient for that matter even if it was there, you'd need verifiable builds and an assurance of no self updating capability too, basically for certain minimal products you need some level of trust. As far as extra verifying though you'd probably start the same way you would minimally blackbox anything: stick it on a virgin test box and monitor all I/O in a controlled setting (VM could work too, though if you're super paranoid you might take into account that can be detected and runtime behavior modified). I guess you could start disassembling it and poking around its memory and such too if you wanted.
But seriously, it's a Mac product, so source access everywhere should kind of obviously be off your list already. How do you know when Apple spies on you? You won't find the source code. And the answer is you blackbox it and mostly you trust that it'd get noticed at some point and Apple would get the crap sued out of them and be investigated by governments worldwide etc etc. Objective Development is a very long standing dev and has put out a far more critical long standing security product that has been used by a lot of sec people for a very long time. Sure they could be compromised in the same way any dev could, but on the lowest critical factor level they're unlikely to be the biggest risk in most users' threat model.