The docking adapters for Gemini and Agena were apparently tested for Gemini VI; there's at least one photo, S65-52015, of the ground test. Gemini VI's Agena "failed to go into orbit" and the Gemini VI-A mission rendezvoused with Gemini VII instead.
For Gemini 8 specifically, NASA Press Kit for Friday AM March 11, 1966, Release No 66-52, Gemini 8 Launch Set for March 15 tells us
The Agena and its docking adapter were shipped to the Kennedy Center Jan. 21. These two components and the Gemini spacecraft were mounted atop a 50-foot "Timber Tower" at KSC'S Radio Frequency Test Systems site Jan. 27-28. The prime Gemini 8 crew and their backups boarded the spacecraft on the tower to conduct a series of Radio Frequency Capability Tests between Gemini 8 and the Agena target. Docking compatibility checks also were made between the two vehicles.
Emphasis mine. It isn't clear to me if the "Boresight Range Tower" of the Gemini VI photo is the same as the "Timber Tower" referred to in the press release.
All found by Googling "gemini agena interface test."