Inspecting intermediate values in a geometry node tree is useful while building/understanding one or when trying to figure out why something is not working. Blender provides multiple tools to understand how a node tree is working or why it is not working.
Generally, the inspection tools display data from the last time the node tree has been evaluated. If it has not been evaluated, no information is available.
Socket inspection shows information about the value in a socket during the last evaluation. For primitive data types such as integers, vectors, and strings the actual value is shown. For geometry sockets only some data about the geometry is stored, including the set of data types the geometry contains, and a count of their elements.
Socket values are only logged from when the node tree was executed, so a node must be connected to the group output to have a value for inspection. Values are not logged during rendering, to improve performance.
The attribute search is shown when clicking on an attribute input in the modifier. It contains a list of all the attributes that were available at that point in the modifier or node execution.
The Viewer node is used to display intermediate geometry in the Spreadsheet. For more information see Viewer Node.
When the inputs to a node are invalid, it displays a warning in the title. Hovering over the warning icon shows the error message. These warnings are only generated when the node is executed, so a node must be connected to the group output to have a warning.