Hello u24c02,
As ads-ee has explained, I would also like to say that it is like a chain-of-trust which remains unbroken.
It is a rare case that you will get an untrusted netlist for Dft insertion. You see this is why softIPs are so popular. A fabless company in most cases will buy softIPs and then the internal design team will proceed to synthesis, dft, etc.
"How can I believe the result of synthesis netlist from someone?"
Well you ought to have some reference point. The reports of the first scan-inserted netlist becomes your 1st reference. Now suppose there is a change in the RTL and a scan-inserted netlist is again generated along with the reports. You can now compare the new reports with the old reports.