Oh, its a pretty easy thing to spot....your betta will look like hes swallowed a marble whole. Often this is accompanied by lethargy, stressed breathing, and difficulty swimming either sitting on the bottom of the tank or floating up at the top partially sidewise(which would then be SBD, Swim Bladder Disorder, as well which is often the result of bloating, generally caused by over-feeding).
Bloating on its own is often cured simply with a few days of fasting, SBD would require a little more attention with an Epsom salt treatment and daily 100% changes in a smaller hospital tank along with fasting.
Are you worried your betta might be bloated? What do you feed and whats your feeding schedule?