You know your own horse better than anybody and if you suspect discomfort is stemming from the mouth a process of elimination will usually determine this. Firstly an MOT is necessary.
Examine the mouth yourself – please refer to the section on ‘Mouth Management’. Also have the teeth and mouth checked by a qualified equine dentist. Some dentists may want to see your bit in situ. Bear in mind no matter how experienced your dentist is some conditions can only be diagnosed with x-rays, scans etc. Have the back, neck, poll etc checked by a qualified physiotherapist who may wish to see your saddle fitted. Have a master saddler check your saddle, some of whom will want to see you sit on the saddle in situ and ride your horse. Farriers play a vital part, are the feet OK and balanced. Seek veterinary advice, your vet may wish to see the horse lunged and perform a flexion test. Evasions can come from behind and manifest themselves in what seems like a mouth problem. Seek advice from Horse By Horse, who can set you up trial bits.
Another test would be to use a hackamore such as an English Hackamore or a bitless bridle, but only in a ‘safe’ environment such as an indoor/outdoor school. It is important to follow the same program as when you are using the bit, but bear in mind it is sometimes more difficult to obtain the same shape/outline in a bitless bridle and it is not always as easy to set up the bend. However, this must be achieved otherwise we cannot fully evaluate this experiment. This is because the horse will not be performing to the same physical standard as when you employ the bit, so if it is a muscular or skeletal problem then we won’t be able to make a valid comparison.
A check for saddle discomfort is to ride bareback, however this is not always an option, especially with our high withered thoroughbreds and gentleman riders! If your horse is safe to ride bareback a neck strap is always recommended. Again you need to be able to achieve the same way of going in order to evaluate this.
Is your horse good to lunge? If appropriate side reins may be attached to the lunge cavesson using a roller or surcingle in order to achieve an outline on both reins. Make sure the horse has achieved a decent shape and is coming through. Then repeat this process with the side reins attached to the bit. Alternatively, you may use a Pessoa lunging aid if your horse is familiar with this as it allows a more natural movement of the ‘reins’ attached to the bit, as apposed to being fixed with the side reins.
The horse may also be lunged fully tacked up. Do use an overgirth to prevent your saddle from banging up and down on the horses back and have the reins knotted on the neck but not acting, and lunge on both reins. Then put a rider on board with the reins in the same position. As a safety measure slip a flash through the D rings on the saddle for emergencies so the rider can remain upright instead reaching forwards for a neck strap. The rider must resist the temptation to pick the reins up. The horse may initially react anticipating rein pressure, but slowly relax. However, if this does not happen it may indicate a back problem as the rider’s weight is the only thing that has changed.