i have a garmin forerunner 45S. is there any test to identify your HR zones?
Garmin has their own Lactate Threshold(LT) Test which can get your estimated LT HR and Pace. Then you use Threshold HR for zoning instead of Max HR. This would be the most practical that give you a close enough way to zone short of doing the actual Lactate Threshold or VO2Max test in a Laboratory($$$) to get your LT HR and Max HR respectively.
But I don't think your model has it. Only the mid range and higher end models and it requires an external HR chest strap monitor.
You can google how to do LTHR test. There's a few variations on the internet, most would require you to run your lungs out for around 30min.
Some watch like my Coros Pace 2 or if you upload your runs to Training Peaks, they have auto LTHR detection after some number of runs. However this may not be as accurate as the actual LTHR test.
Once you have your LTHR, just set this HR into your zone setting in Garmin and remember to change the calculation method from default Max HR to LTHR.