01
Describe what is actually present
Note the location, direction, width, texture and continuity of a ribbon or inclusion. Compare dry and wet appearances and review whether it intersects nail holes or creates a plane of weakness.
02
Use source-specific experience
Different deposits contain different minerals and structures. Historic field performance and product-specific testing are more useful than a blanket rule applied to every visible line.
03
Sort questionable pieces
Do not install a piece with cracks, weak edges or suspect zones in a critical location simply because most of the slate looks sound. Set it aside for supplier or project-team review.