Depends. Sometimes nothing. But symptoms could be any of the following:
*disinterest in food
*trying to eat but "missing" the food
*inability to get the food down/spitting out food
*"coughing"/flexing of the trigger
*itching/twitching
*not putting on weight even if they are eating
*white, stringy feces
*heavy breathing
*lesions, lumps, granulomas, white spots, bubbles, and other external manifestations of parasitic or bacterial infection
*whitening of the tail, whitening of the snout, erosion of the tail and/or snout
*erosion and/or fraying of the fins
*lethargy
*buoyancy issues (either sinking or floating)
*webbing of the skin/on the skin
*death, could be sudden and without warning
But, of course, some of these things could manifest later down the road. What I'd look for in the seahorses when you purchase them is having good size to them, no smaller than 3 inches, broad chests, filled out abdomens (space between body rings should not be concave), and high activity level, especially when feeding.