When I do an evaluation I start with really easy questions. Questions are based on certain levels.
For example
What is your name?
Where are you from?
Are you married? or if younger, Do you have brothers or sisters?
Do you have children?
How many children do you have?
How old are your children?
Do you have a pet?
With a series of these type of questions I am able to see if they know the verb to be and to have.
I then ask questions regarding Simple Present Tense. I normally ask for their routine. With this I am able to see if they know the vocabulary for a routine and Simple Present Tense.
I have them ask me very simple questions to see if they know question formation.
I then continue with other tenses. If they are able to get as far as the Past Tense I have them tell me about a vacation they took and what they did on the weekend.
I continue like this with the tenses and I also do situations.
With this I am able to evaluate their level of English. Hope it helps.