How can you tell if your bra fits?

First thing to do is dump your lingerie drawer onto the bed or nearest surface and try on all your bras.

Are they are too big in the band? Which you can tell if the band is riding up your back, it's loose on the tightest set of hooks or the elastic has no snap left in it.

Are the cups are too small, you are spilling out from the top bottom or sides?  We have compressible flesh and the band needs to be snug to properly support the bust, which will leave indents but that is normal in a properly fitting bra.

Is the bra is pinching, jabbing or uncomfortable in any way?

Return the bras that fit and are comfortable to your drawer.

Your bra should be snug but not painful. Every woman's definition of snug is different but you should be able to flip the straps off and your bra stays up. The underwires should sit at the crease between your breast and body. So that when you lift you arm up the underwire stays in place and your breasts do not fall out the bottom. The straps should have room for one finger width at the shoulder. If the straps are too tight, presumably to get lift, the back will ride up and the cups will sag. the cups should encase your breasts so there is a smooth transition from the bra to your body. Thus avoiding the quadruple boob.

All of these criteria must be met for your bra to be deemed a proper fit. There are many size combinations and styles. you may want to try on a variety and compare the fit.