How can you know if you are infertile?

Consult an MD. Basic testing can be performed by your obgyn, more advanced testing/treatment is usually done by a reproductive endocrinologist. If there is male factor, usually urologists will work-up the male.
Your history. One should seek help after 1 yr of trying. If the woman has regular cycles and is unable to conceive after 2-3 years, either her husband has very poor sperm count or motility or she has serious problems such as tubal disease, pelvic adhesions, or endometriosis. Most couple with 3 yrs of infertility will eventually need ivf. Women whose cycles are > 32 days will need medications to ovulate better.
Time. Infertility is failure to conceive after 1 year of intercourse and attempted conception. If the female partner is over the age of 35, then 6 months duration is used instead.