I can pretty much any time. hope it doesn’t mean anything too bad.
Yes it does. Just like if your hand is the same size as your face it means you have cancer.
I can see it better in my stomach as I watch it pulse with each beat.
Nope, unless you have an inferior aorta with very high blood pressure or you heart just has a very high cardiac output/stroke volume you wont see it beating in your stomach. That is the peristalsis of your gut as it moves your food along your gut.

If you are thin, which usually means you are kind of fit, you should be able to see your heart beating. Other than skin and the sternum, there isn't much in the way.