First assess your max. You need to really tax yourself to get it. Warm up sufficiently and then go up a long, gradual incline. Start with a cadence of 90-95 seated and use a gear where this effort is moderately difficult to maintain. Do that for a couple minutes. Then shift to the next gear and keep the cadence the same. Do it for a couple minutes and shift again. Keep that process up until you can't maintain the cadence and you are struggling. Then shift one more time, stand up, and sprint all out for 15-20 seconds. That will be close to your max. Also it's good to max your HR monitor track max because its sometimes hard to see numbers on a monitor when you're at max.