Joined: Jan 2009
Posts: 44,185
Likes: 6,421
From: New York, NY, and High Falls, NY, USA
Bikes: 1962 Rudge Sports, 1971 Raleigh Super Course, 1971 Raleigh Pro Track, 1974 Raleigh International, 1975 Viscount Fixie, 1982 McLean, 1996 Lemond (Ti), 2002 Burley Zydeco tandem
I think it is important not to aim bright lights into people's eyes. [MENTION=227213]wphamilton[/MENTION]'s method is pretty good. Or you can do it outside. See where the top of the bright part of the beam lands 30 or so feet away. Make sure that is below people's faces.
Alternately, you can get a light with a shaped beam, such as the lights made by Busch & Müller. It's really easy to see the top part of the beam as it is the most intense. The cutoff is very sharp.