Signs of not able to have kids?

Not many. Many, if not most people who need help getting pregnant only find this out once they start trying to have a child! there are a few obvious tipoffs, like irregular/absent periods, childhood hormonal and/or anatomical problems, and a few others that can be quite obvious. But otherwise, there aren't many "outward signs" of infertility. Good luck!
Not necessary. It is not necessary to have obvious signs that would be indicative of infertility. Most infertile women do not have any outward signs that would point towards infertility. Having said that, some signs/symptoms that may be a cause of concern for conceiving include irregular menses, heavy bleeding/pelvic pain due to large uterine fibroids, extremes of weight (obesity, malnutrition).