Flat triangles and edges can be located in a number of ways in ArcGIS.
- Display the original contour lines or the TIN soft-edges over the TIN model and look for triangles and edges in with nodes derived from contour lines with the same elevation.
- Use the Information tool to explore how values change within a triangle or along an edge. If all values are the same it is flat.
- Display the TIN in ArcScene and look for "flat looking" triangles. Confirm with the information tool.
- Render the TIN to show slope. In the TIN Symbology tab Add... a Face slope with graduated color ramp renderer. Now change to symbology levels to differentiate triangles with very low slope from those with a slope.
The red triangles are flat or very nearly flat.