Is it possible to establish which plug-ins are actually being used? I am looking

I think that would be a great feature to have! It would be good if there was a programmatic way to check if a particular plugin or class was properly loaded too. We have a health check process that runs and checks engines, database connectivity, etc. and I have been wanting to add a way to validate that each plugin we are intending to use is loaded in the system. We can see it visually via the /admin interface, but an automated process that checks would be helpful. There may be a way to check usage & proper installation of plugins but I haven't investigated it enough yet to find out.

I am not aware of a current way to see last time a specific plug-in was used programmatically. I have provided this use case to Appian's engineering team. I have had the need before to do this and the way that I did it was to remove the plug-ins from a UAT environment, test and add ones back that were still necessary.

The health check output contains a few .csv files that can help you with this. Run a health chech and take a look at hc-plugins.csv and hc-process-nodes.csv. That will at least get you the smart services. Not sure about expression functions though.