See, that's the thing- they sag in a good way, but sometimes- for example, in the DivX movie from mc-nudes.com- you can see double creases, possible scares, or a lumpiness, if even for a moment, that makes me doubt.
I'm not saying it's impossible she's all natural, or that breasts on a frame like that are impossible (I still believe Stephanie Dahl is all natural too!). Just that there are some tell-tale signs...