It's not about the incline or the speed - it's about your heartrate and your intensity levels.
During the "high" part of your HIIT, you should be working at your maximum intensity - really sprinting it and pushing it hard. You actually want to "feel the burn" here to indicate you've hit the anaerobic part of your workout. Then your "resting" levels, your rate should drop back to 60% to 70% of your max.
I will say that one of the best things I ever bought for my workouts was a Polar heart rate monitor. I highly recommend one. But if you're not ready to make the investment, then try to at least check your heart rate on the treadmill and get a feel for what each level of intensity feels like .. so you'll be better able to judge.
Oh, and if you finish a 30 min set of HIIT and you're not sweating like mad and have trembling legs ...
... you're not doing it right.
.