There's no one thing to look at to tell monitors apart. You just have to examine it, look for identifying marks. Some monitors have a paper label glued to them with a model number. Many don't. Sometimes the labels are missing. You just have to look at things, like the position of the flyback transformer, layout of the controls. layout of the components, markings on the board, etc. It's like being able to identify the model and year of a car by looking at it. Once you become more familiar with lots of different monitors, you can spot them from across the room.
There are about five really common color raster monitors. Electrohome G07, WG 4600, WG4900, WG7000, and the Sanyo 20EZ. There are lots of other monitors, (Matsushita, Kortek, Sharp Image, etc.), but those five are probably the most common. They're all unique looking, get to know those first - you'll probably see a lot of them.
-Ian