The stomach is located in the upper half of the body, between the ventral fins and the start of the anal fin, you should be able to see the anus.
With bloat-the stomach area looks like they swallowed a marble
With bloat and constipation issues the fish will usually be symptomatic, you want the stomach to have a round look but not too round like they are pregnant
Most common symptom will be no appetite, lethargic, floating or sinking, side swimming, with constipation they are often bloated and not having bowel movements as well.
With bloat and constipation it can often also cause swim bladder problem due to the duct being clogged-prolonged bloat and constipation can lead to secondary problems with swelling and infection at the duct and swim bladder
Treatment when caught early-what I do- is to hold food for 2-3 days and water changes, if no relief from fast and water changes
I will feed live foods-mosquito larva or daphnia for a day or so and then if that is not effective.
I use Epsom salt 1tsp/gal along with daily water changes until asymptomatic but no longer than 10 days in salt tolerant fish.
Some people also have good results with feeding de-shelled pea pieces and soaking food-I do neither-but many have and swear by it.
Best way to prevent-regular water changes and not over feeding, removing uneaten food after feeding.
Remember the stomach is about the size of their eye, feeding small frequent varied meals is often best.