How to know if errors are due to linear or non-linear issue in an optical link during troubleshooting errors?

When the bit error occurs to the system, generally the OSNR at the transmit end is well and the fault is well hidden.

Decrease the optical power at the transmit end at that time. If the number of bit errors decreases at the transmit end, the problem is non-linear problem.

If the number of bit errors increases at the transmit end, the problem is the OSNR degrade problem.