The symptoms are quite distinctive. Your horse may be a headshaker if it exhibits any of the following
A sudden twitching of the head as if stung by a bee on the nose
Hiding its muzzle by over bending, in corners, in water, in other horse’s tails and in shrubbery
Rubbing its nose on its forelegs on objects or along the ground
Striking out with its forelegs
Snorting and sneezing more than normal
Flipping or waggling the nose/top lip or clamping the nostrils closed.
If you are at all unsure please contact your vet for advice.