To get the most accurate numbers, you will have to go in and find them over 3 workouts this week. (E.g. Monday 15RM numbers, then your 10RM numbers wed, and your 5s on friday...then take 9-14 days off and then start your cycle). To find the numbers I recommend a general warm-up (e.g. 5 minutes on a rowing machine, skipping, etc., then some light stretching. Then to ramp up just start really light for 15-30 reps checking your form is perfect, wait a few minutes, adjust to what you think you can do, and test what you can actually do. For the 5RM's you will want to ramp up the weight slowly with a couple lighter sets first, but you should have a good guess of what it will be by then.