When you are trying to discern an Enneagram style in yourself or others, here are some dead-ends you might run into.
First, you might assume their style is obvious. Now, in a few cases, their style leaps out at you. If it does, you might be a bit apprehensive, because our Enneagram style is often neurotic; it is a coping mechanism that has roots in our childhood, do beware.
Second you might make your judgment on one or two characteristics. Many Enneagram books discuss the styles in terms of "traits," so if you see one of those traits, you think you have the style. "Oh, she has all her credentials on her wall, she must be an image-conscious Three." Or "she offered me a glass of water when I came to her office, she must be a Two."
Third, you might see unusual efforts to solve problems because of an unusual situation. Under punishing deadlines, the most light-hearted Seven can look like a Three, and if a Five gets upset enough, he might look like an Eight, especially if he is in an argument.
Your mistake would be ignoring context.
When you try to determine your own style, go back to when you were 20. And make sure you picture yourself in a variety of situations. In graduate school, everyone has to be a Five to survive, but what about your preferences and behavior in other situations?
Another mistake I call snapshot. Look at your behavior over a long period of time, not one dramatic moment or one specific situation.
This is subtle, but people often look at behavior instead of energy. A Six and an Eight both can get angry, but you need to calibrate the energy. An Eight is much more apt to be a feral force whereas a Six gets over the anger quickly and often will back down in a fight.