Testing metadata plugboard function

[Phssthpok]

I'm trying to test a metadata plugboard function for formatting author names using author_sort, but I can't see any way of testing it in the metadata plugboard dialog, and the Template Tester plugin only seems to let me process the book title.

There must be a way to do this, so I'm obviously being dim. In the meantime, the only way I can test it is to plug in my device, upload a book, unplug, check the author name, rinse and repeat, which is incredibly slow and tedious.