Typical loose ball (or retainered) headsets are angular contact type bearings. They consist of a conical (inside) race and a cup like (outer) race. (Notice I didn't say upper and lower, because, while the cone is usually the lower component and points up, some designs are inverted.

The above applies to each bearing, so it repeats on the upper and lower end of the head tube (see picture here)

Now lets look at how it would work if the cone points up. The ball contact area is the lower inside of the retainer, and the cup touches the upper outside area. So the metal ring of the retainer has to be to the lower outside or the upper inside otherwise it would get in the way. The fingers that hold the balls don't count because they're actually between the balls and wouldn't touch anyway.

You might drop a retainer over the cone, and and hold the cup section and spin it (without the fork) to see if you can feel the the retainer rubbing.

Or try this. Roll a sheet of stiff paper or cardboard to form a cone (like a party hat) but as shallow as possible. Drop the retainer over the cone, and see if the balls can touch the cone with the retainer touching. Flip it over and see how that works out.

You can also flip the cone over and try the same experiment on the inside, looking again for balls to touch but not the retainer.

Once you can visualize the areas where the rolling contact happens, and those out of the way, you'll know which way the retainer has to be.