Find Trending topics without background set

Is there an independent metric to verify if this trending topic algorithm is working correctly ?

I wanted to check the returned trending topics against the IDF scores, but there is no API to access them. Elasticsearch does return term vectors but there is no way to obtain IDF scores (as discussed here Accessing tf-idf)

My problem is as follows : I have not seen other trending topic algorithms use a background set explicitly, so I am reluctant to just deploy this as-is.

I see that I can use the background filter to change the background set. That would provide some proof via experimentation that the trending topics are changing as I vary the background set. Would you recommend that ?