Easy to tell by the resolution. ie 20" and 22" screens are 1680 x 1050 so divide 1680 by 1050 and you get 1.6 = 16:10
24" screens are 1920 x 1200 which again equals 1.6 therefore is 16:10
All computer widescreen monitors tend to be 16:10 as far as I know but some tv monitors are 16:9 but I'm sure somebody will correct me if i'm wrong