This is a really great portrait photography trick that requires no special equipment: use a black marble to see how the light hits on the eyes of the person you want to photograph:
Just hold a fist in front of you (like holding a telescope), tuck the marble just under your forefinger, and there you have it – the same lighting an eye would get.
And since you know you want the catchlights to be up at 1 to 2 o’clock, or up high at 12 o’clock, simply turn about until you see the catchlights you want.
The neat thing is that the curves and wrinkles of your hand show you the amount of contrast and backlight.