The basic rule for determining if a girl's boobs have an overlap of boob and chest due to large size, or due to aging, aka "sagging."
The rule is as follows:
If, when laying naked on her back, with arms stretched up, there is overlap of boob and chest, then said boobs sag. If not, then any overlap when standing is due to large size.
-Dude, is that chick sagging?
-Nah man, I got with her the other day. She passes the sag rule. Those boobs are just huge.
-Good call dude.