It doesn't need to be that hard. If you can converse easily while running, you're going slowly enough to be in the fat-burning zone. Or you can buy a Heart Rate Monitor. The new ones let you program 70% of maximum heart rate. If you go too fast it'll beep annoyingly. It will also do 90%. If you slow down it'll beep.

For doing 90% on the treadmill, I crank the level up to 12% grade. Then I try 400 yard sprints with one minute rest breaks. If you can barely catch your breath and can feel your heart hammering then you're probably at 90% of max heart rate.