Me and my boyfriend would use latex condoms. Before him, I wasn't really sexually active so there was no possible way that I could have known that i was allergic to the condoms. Every time after having sexual intercourse, my lady parts would get itchy. They would also burn, and occasionally get a little swollen. This would only happen after sex. Luckily, the reactions never got severe enough for me to go to the doctors.
The way I found out was I researched it on the internet and I also had a friend that was going though the same thing. I talked to my boyfriend about it and he switched to latex-free condoms. Those symptoms never came back again after we started using latex-free condoms.