You should be able to hear your horses’s gut sounds as your horse digests his food by putting your ear to his abdomen. You need to do this on both sides of your horse. In a horse these noises should be nearly constant, as horses are trickle feeders and therefore should be eating roughage like hay and grass consistently throughout the day.
If you can’t hear the sounds or the sounds are much quieter, reduced or different to normal it could be a sign of colic. Alongside this you are likely to see some of the other signs of colic, like pawing, rolling, kicking the abdomen, looking at the flank etc, but this is not always the case. Either way, colic is an emergency, and you need to put a call through to your vet right away.