There is a rule of thumb that anything below 80%HRMax is working within the aerobic zone, whereas above you are heading towards threshold and anaerobic zones (being able to sustain these harder efforts for less and less time).
Personally, when training aerobically, I aim for roughly 75% and not higher, even when it comes to hills. There is thought within the coaching communities that if even a slight fraction of the run goes above the aerobic zone threshold, then the aerobic training effect is greatly diminished.
As stated by Julii above, there is no easy way to determine your own heart rate zones, lactate threshold etc, as they are all based on rules of thumb, and averages, and the fact is that everybody reacts differently to the exercise stimulus.
However, there are options available, depending on how keen you are to determine your true zones - a wide number of sports institutions carry out VO2 testing, and with that they will help you determine your heart rates zones. This isn't cheap, however, and I imagine a good 12 months training and you would need to have undergo testing again.
For most casual runners, the algorithms carried out by Garmin, Strava, TomTom etc., will be good enough. Whilst these seem to be arbitrary to begin with (as in fact, they are often just percentages of your HRMax), you can start to adjust them by running by feel - which is something that seems to be lost nowadays with the masses of information available at our wrists.
If you are interested in a slightly more involved method of determining heart rate zones, and/or heart rate zone training, I point you in the direction of Hadd's Approach to Distance Training. If nothing else, it makes for a very good read for the more competitive casual runner, that is looking for a different way to train [I had started following this training method in November, just before I broke my collarbone, but intend to embark on it again!]
I hope this has offered some insight, and I wish you the best of luck!