You can try with a heart rate monitor for most intervals over 5 minutes. Under that, you need to be in tune with your RPE. Read up on it
here. One technique that helped me a lot before I had a power meter was to go do negative splits on hill repeats, measuring time. Once I got to the point that I could finish my 5th ~five-minute interval, all-out, and only be about 15 seconds faster than the first, then I found that I could gauge my efforts really well in other settings.
At any rate, link the recipes back to training zones (using the link above) and convert those to RPE and/or heart rate. You'l be fine. You don't have to nail it perfectly. It still comes down to "time in zone." A power meter is helpful though...