There are many signs that indicate if you are over breathing such as frequent sighing or yawning, difficulty sleeping, over eating, 'forgetting to breathe', and lack of energy, to name but a few. Learn more in this article
But a good test is that unless you can comfortably pause after a normal sized OUT breath without needing to breathe for 45 seconds or more, and then take only a normal breath IN - not a big gasp, you over breathe according to Professor Buteyko.
Most people do.
If you over breathe it is likely that your horse will to some extent copy this and vice versa. It is easily possible for horse and owner to get into a spiral of deteriorating breathing patterns.
Since over breathing causes symptoms and problems this is not a good situation.
From your point of view, over breathing may exacerbate any chronic conditions such as asthma, migraine, allergies, fatigue etc. Your muscles may be stiffer and tenser than they should be, you may find yourself short tempered and irritable and have difficulty concentrating.
You may feel your fitness is not as good as you would like; you run out of puff easily and tend to ride with an open mouth.
Competing can be exciting and sometimes stressful and this encourages over breathing. Many riders find they have to breathe through their mouth when they compete. If you are one of these riders you may be reducing your own and your horse's performance.
In difficult, stressful situations the handler often talks to the horse thinking that this calms the horse. But people often over breathe more when they talk than when they keep their mouth closed so over breathing problems such as tension and irritability are increased. Horses don't understand the meaning of words, but they can pick up on tension, irritability and anxiety in the voice and these are not reassuring. So sometimes keeping quiet can be more helpful.
Soothing sounds can of course be helpful - especially if they prolong the out breathe. This favours the speaker's parasympathetic nervous system which encourages calmness and this transmits to the horse.