NodeCanvas has a rather informative and advanced runtime visual debugger. In play mode and while a behaviour is running, you are able to see exactly what’s going on within the graph editor window, where nodes and their connections will get highlighted with specific colors to indicate their status. In the context of behaviour trees, these have a more specific meaning as follows.
Also remember, that you can Live Edit the graph while it’s running as much as you wish. Adding nodes, connecting disconnecting etc.