You would have to ask the participants of the Zcash trusted setup ceremony.  All of them would have had to collude to keep their keys.  There are many questions about the trusted setup... but at some point you have to trust that it was done properly.  The only way you would know for 100% certainty is if you participated yourself.

There were questions though, so Zcash might do another more elaborate ceremony.  This will not affect Zencash, Zclassic, Hush, and all the rest, as they were forks from the original.