It is very difficult to tell but what I usually look for in mine is an overall flatness to the bottom of their belly and they will usually be pretty big/ swollen out on the sides.
Some people refer to this as looking like they are about to "pop" they are so big.
ueah, it's tough to see here but I would venture to say she's not close enough to be separated into a breeder. Her body will square out sort of and the dark spot will be further down near her bottom fin...