How to determine the ground with a multimeter on the pins of the video card?

Question

HDMI

The ASUS GEFORSE GTS250 graphics card has two pins, labeled J10. on the forums they write that this is SPDIF IN. How do I know which of these pins is ground?
The purpose of this event: I need to connect the video card to the sound card so that sound via HDMI works.
If possible, then in more detail: this probe - there, this probe - here; it should be on, it's off. Thank you.

Answer 1

Try one probe on the card blank, the second - to call contacts J10.