Troubleshoot Nodes with Interrogator

Description:

Troubleshooting a node setup can be difficult because the values inside a node group have a different value for each pixel. There is no 'one value' to print out. Interrogator helps you visualize an Input Value by tying to a node socket in your node graph and adjusting a Compare slider. The render will show black and/or white material.
The border between the black and white is where your value equals that in the Input value.
This is far less tedious than it might sound and it is enormously insightful to see the whole render change at once while you change one value; give it a try!

Use:

BTW, not all shaders will show up in the Material viewport mode so you should use the Render preview mode to get the correct behavior.
See the setup picture above, or use these instructions:

You need to modify your node graph slightly:
1. Add a Mix Shader to the final output and tie your own node graph to the lower BDSF input.
2. Add the Interrogator node and ties its 'Test BDSF' output to the Mix Shader upper BDSF input.
3. Tie the 'Display' output to the Mix Shader Fac socket.
4. Now move the Interrogator node group anywhere you want and tie any node to the Input socket. You can toggle between the testing render and rendering your own material by toggling the OFF-ON slider.
+ Adjust 'Glow Factor' for dark situations.
No magic here, but I can't even imagine a better tool for troubleshooting a node graph.