Probably the simplest way is to use the 'Add Output Variable' BCL measure, entering the node name as the key value and one of the 'System Node ...' output variables from the .rdd file as the variable name. You can add multiple instances of that measure in the measure tab, to track what's going on across your loop.

I think there are two parts to your question 1. How do I get the Node's handle? 2. How do I add variables to see behaviour at that node Eric has answered the second part

I'd be tempted to do this sort of troubleshooting in e+ because I find that quicker and more direct to add output variables and inspect csv results that way. On the other hand, I wouldn't do that in e+ if I had to repeat it, because the .idf gets clobbered when you save the .osm

You can get the node name from the HVAC page, like this image node name.JPG