I bought them for like $5 on amazon, they’re typically used to detect if you’re starting to go through menopause but conversely they can apparently detect if you’re follicle stimulating hormone levels are on track for fertility. See below for amazon description:

Female Fertility Test

As I am sure you are aware a woman's fertility starts to decline after she gets to 30 but what you probably did not know is that the hormone F.S.H. (follicle stimulating hormone) is intimately involved with a woman's fertility and rising levels tend to be associated with reduced fertility. F.S.H. is the hormone that maintains the regular function of your ovaries and sex hormones and is a useful aid to assessing your fertility.