Threshold changes day-by-day and, with training, improves week-by-week and month-by-month. The only way to know, and “know” is a bad term to use because it is a changing value, is to take a lab test aimed at finding the threshold.
Athletes have to learn to feel the threshold as they cannot get tested everyday. The test, as well as using a portable lactate tester in training, serves to reinforce or confirm what they feel their threshold is, or what they feel their easy pace is, etc. Recreational skiers can get tests at university laboratories or sports centers for very reasonable prices.
If they aren’t interested in this, they will have to use a formula and/or go by feel.
It’s a comfortably hard pace that can be maintained for upwards of an hour and a half. Formulas are not accurate but may give you a start. A skier’s threshold is often between 80 and 90% of max (and even higher). Wear a monitor and, starting slowly, build up your pace gradually paying close attention to your breathing and heart rate. When your breathing is hard but rhythmic and in control and you feel taxed but as though you could go for a good long while then you are probably around threshold. When your breathing becomes a bit ragged and just out of control, and you feel that you could not go for very long then you have crossed over your threshold. Note your heart rate all along the way. The heart rate where you are running a bit ragged is above threshold, so error low. It can be the case that you have predicted your threshold at 175 one day but are running ragged at 173 another day.
What you hope is that you notice the running-ragged-heart rate creeping up. If it is going down, then you know you are training too hard, too much, and/or resting too little. It is a flexible value, so don’t think that this can all be boiled down to some numbers. You will have to be involved in deciding for yourself how fast to train regardless what the heart monitor tells you.
Don’t make it too complex. Easy feels easy, hard feels hard… tired feels tired. Trust what you feel, and train well.