Here are a few signs, but if you feel that your horse may have issues the sooner you get help the better off your horse is:
(taken from Laminitis - Wikipedia, the free encyclopedia
Increased temperature of the wall, sole and/or coronary band of the foot.
A pounding pulse in the digital palmar artery. (The pulse is very faint or undetectable in a cold horse, readily evident after hard exercise.)
Increased vital signs and body temperature
Walking very tenderly, as if walking on egg shells
Repeated "easing" of affected feet
The horse standing in a "founder stance" (the horse will attempt to decrease the load on the affected feet). If it has laminitis in the front hooves, it will bring its hindlegs underneath its body and put its forelegs out in front called "pointing"
Tendency to lie down, whenever possible or, if extreme, to remain lying down.