Automate fault localisation / demarcation

I’m looking at ways to reduce the time and troubleshooting it takes to localise a fiber DWDM client side fault to the customer equipment or customer cabling v the DWDM equipment / cabling. What are the behaviours or alarm patterns to indicate an issue on the customer side alarming on our DWDM monitoring equipment? Same for errors on DWDM client facing interfaces -we struggle to quickly pin this down to the customer or our DWDM equipment without lengthy troubleshooting