I attached the side labeled output to the HDMI port of a computer's video card and the input to the TV.
50% chance of getting it right. Some are labelled "source" at the source end and "display" at the TV end. Harder to misinterpret that one.
With "input" it could be the input end of the cable or it could connect to the input jack on the TV.
Short HDMI cables are symmetrical and bidirectional.
Inexpensive long cables are too, but they have a length limit.
Expensive long cable, which push the 60' or so limit for "passive" cables have active electronics which amplify and contour the gain with frequency to get the best possible high speed digital signal out the other end. These are totally directional.