I guess it seems to come with experience and knowing your snake. If your snake is the size/age but you just don't think she's ready--maybe she's been going off feed, maybe she's just seeming a little off..Maybe you just want to wait til she's for sure old enough. I guess when in doubt, don't breed. If you know your snakes and know they are strong, a good weight, and healthy, than I guess it's up to the individual owner to know what is safe for their animal or not.