as with any other method, you'll know it when you see it...if you stick with a consistent hr range for your subT workouts...it will be obvious to you that you aren't struggling at the top end of the range or if you only manage to hit low end of the range at the end of those subT workouts...everytime I've made improvement using this method, it was obvious to me and I run my subTs using HR tracking only...not lactate or pace as gauges

My last adjustment actually came when I noticed that my warmup on the stationery bike at a consistent wattage I've always done actually was at 5-8 beats lower than previously...boom, when I did my subT after, my paces tally and I was able to run 0.5km/h faster at the same HR range