Keithpenny
It means if there are 200+ O2 masks laying loose then it's a good chance it was a decompression. If most of the intact PSUs still have the O2 mask door close then there's a good chance cabin pressure never exceeded 14000msl. Quite simple really.
It is highly unlikely that any primary failure would cause the masks to deploy as a secondary failure and few failures that will result in a loss of cabin pressure. What I am saying is it will be very easy to rule a loss of cabin pressure in, or out.