Can one check if an Elasticsearch index really uses best_compression after adding a corresponding template with best_compression and forcing merge?

Question

How one can check what compression type is really applied to an index in Elasticsearch? I had an index that I assumed was compressed using default method (LZ4). I closed the index then created a template for the index with best_compression codec. Then I opened the index back and forced it to merge setting max_num_segments to 8. The index had 17 segments originally. The size went from 5.8 to 5.7 gb. Now I am curious if best_compression really applied.