Create at least three basic English tests, all related to one another. Depending on how many students you have and the number of years with which you work for them, you will need a series of progressively more difficult English tests, measuring all basic skills (reading, listening, speaking and writing). These should have questions that are similar enough to gauge progress, but using progressively more difficult vocabulary and sentence structure. If you want to keep very close track of your students' progress throughout one particular year, you can give them different versions of the same level test every three months. Evaluate the tests and keep careful records of the grades.
Have students send you recordings. If your students mostly work on their own, but you wish to still monitor their progress, have them record themselves in MP3 formats and email you the results. Provide them with a prompt, such as requesting their opinion on a current local event, on which to speak, and take notes on your overall impressions, as well as more detailed pluses and minuses from each recording.
Keep a file on each student with his or her information. With your notes on their progress tests and their recordings, you will need to assemble this information to have a good idea of the students' progress. You can create an overarching evaluation for students that you assess each week or month, measuring factors like their vocabulary, sentence structure, spelling and accents, that you make and keep in this file, based on your earlier observations.
Meet with each individual student once a month. Not only will meeting with each student help you to see his or her progress first-hand, but it will also give them a forum in which to ask questions, provide you with feedback and speak one on one with a native English speaker. Take notes on your meeting and keep them with your students' file, possibly filling out your evaluation each time you do so.