The short answer for a working Standardbred
A Standardbred who is short of fluid usually tells you at the mouth, the skin, and the manure pile first. His gums feel tacky or dry rather than slick, a pinched fold of skin on the neck or point of shoulder is slow to flatten, his eyes can look slightly sunken, and he passes less urine, which looks darker. His manure balls are drier and harder than usual, and a horse who has just trained may stay flat and uninterested long after he should have brightened at the wash rack.
Why harness work hides the problem
Standardbreds trot or pace mile after mile, and that steady work produces steady sweating owners can underestimate because there is no single dramatic gallop to blame. Sweat runs under the harness, dries on the coat in the breeze, and is easy to miss on a dark bay or brown horse. A fit Standardbred may keep pulling through his last trip while already behind on fluid, then crash quietly in the cross-ties. Heat in a poorly ventilated barn, a long trailer ride to a track, and water that tastes different away from home all stack on top of exercise losses.
The hands-on checks, in the order grooms use them
Lift the upper lip first and press a fingertip to the gum above the incisors. Moist gums and colour that blanches under pressure then returns quickly are reassuring; sticky gums and slow colour return are not. Next, pinch and release a fold of skin on the neck and watch whether it smooths out at once or stands in a soft tent. Run a hand over the coat for dried sweat and salt, check how sunken the eyes seem, and look at how much he has urinated and how dry his last manure looks. Finally, compare his drinking with his own habit: a Standardbred who normally dives into a bucket after work but only lips the water today has told you something.
What to do while you are deciding
Stop the work, untack him, and walk him in shade until his breathing settles, then offer small, frequent drinks of clean water rather than forcing large amounts at once. Note roughly how much he drinks, whether he eats hay, and whether gut sounds are present on both sides, because that timeline is exactly what a vet will ask about. Recheck the gums and skin tent after he has had a fair chance to drink; improving signs with a brighter attitude are very different from signs standing still or worsening.
The point where this becomes a vet call today
Call a veterinarian the same day if your Standardbred will not drink at all, if the skin tent stays up, if his gums stay dry despite offered water, or if he becomes weak, wobbly, or unusually quiet. Treat pawing, rolling, looking at his flank, a rising pulse, heavy breathing that does not settle, or a complete absence of manure as urgent alongside dehydration signs, because fluid problems and gut problems in a working horse can travel together and downhill quickly. A young or older campaigner, and any horse who was already off his feed before training, deserves that call sooner rather than later.
Keeping a baseline so next time is clearer
The checks above only work if you know this horse's normal: his usual gum feel on a rest day, how fast his skin flattens in winter coat versus summer coat, and how many buckets he typically empties after a training mile. Jot those baselines down after ordinary days, not just worrying ones, and record any episode with what the work, weather, and trailer time looked like. That written history turns a vague 'he seems dry' into a comparison a vet can act on. Storing those baselines and any vet findings together in his KinsLe Pet Passport puts the whole picture in one portable packet for the track vet, the home vet, or whoever jogs him while you are away.