Write a custom probe for a VI Refnum type. In the diagram of that custom probe, put an Open FP method call. Then probe the wire coming out of your Open VI Reference call.
If you have scripting knowledge, you can go further and have the custom probe open the panel and open the diagram and put a breakpoint on the top-level diagram, or some other point within the diagram of your choosing, so that when the VI actually gets called to run, it'll trip the breakpoint.