The first requirement in assessing someone's Enneagram type is to remember to focus on their overall personality. If you focus solely on how they are in public or in private, at work or with friends... or that day at Wendy's you will likely mistype them. So go over in your mind as many memories/recollections as you can. (Your ability to type them with any accuracy is dependent on your discrimination ability, memory accuracy, and the amount of memory data you have of them in various environments.)

If you are able to ask the person questions here is an example of how to find their type (with some degree of accuracy)?

"Do you consider yourself progressive/intellectual or traditional/instinctual?"

Then ask...

"Do you consider yourself socialistic or individualistic?" (feel free to explain that a socialistic person thinks the external world is more powerful than them, whereas an individualistic person thinks that they are more powerful than the external world.)

If they answered progressive socialist, they are some type between 3w2-5w4

If they answered traditional socialist, they are some type between 3w2 - 1w9

If they answered progressive individualist, they are some type between 5w6 and 7w8

If they answered traditional individualist, they are some type between 7w8 and 9w1

In each of the above cases, that breaks things down to three possible main types? So then you just ask which of those three traits they relate to more (see above chart for traits). In the case of 7w8 to 9w1, you ask the person which word fits them most - strong, energetic, or peaceful. That answer is their main type. Then you ask them to choose which of the remaining two words is more descriptive of them. That answer is their wing type.

About ten percent fall in the middle of all type behaviors and can be best labeled omnitypes. So if you find the person feels they are really in the middle on the progressive/traditional question and the introvert/extrovert question, they might be an omnitype.