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 integer and float the actual value is shown. For geometry sockets only some metadata about the geometry is stored. That includes the set of geometry components and the quantity of elements.
The attribute search is shown when clicking on a string socket. It contains a list of all the attributes that are available in the geometry inputs of the same node.
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.