Look up the individual's grade level or language level and determine the corresponding level of English proficiency before you begin. For example, someone at the ninth-grade-level should have a proficiency of around 65-68 on a standard proficiency exam such as the TOEFL (Test of English as a Foreign Language). On the other hand, someone in grade 11 should have a score around 80 to 89.
Assemble a multi-part grammar, writing, reading and listening exam that corresponds to her level of English study or grade level. Most level-appropriate English exam or textbooks have such tests. Alternatively, just give the individual a practice TOEFL exam to take. Acquaint yourself with the time limits of each section and enforce them as the student takes the test.
Take the student's test booklet and answer sheet and set them aside. Ask the student questions appropriate to his language level, asking him to talk about a variety of subjects. For example, for a beginning student, asking him about his family, friends and interests is appropriate. For a higher level student, asking him about politics, movies and English language books he has read is also appropriate.
Assess the student's answers based on how fluently she is able to answer your questions, the level of vocabulary she uses, how complex or simple her answers and how comfortable she seems talking. Give her a score from one to three: poor, average, proficient.
Check your student's answers against the answer key. Calculate her percentage and use that score as a benchmark of her proficiency. For example, anything from 55-65 percent marks a weak proficiency; 65 to 75 percent marks an average proficiency, 75 to 85 is slightly above average and 85 and above marks a high proficiency.