take your temperature before you get out of bed or eat or drink or smoke each morning. graph your temperature on a chart. when temp is rising, peaking and falling are all considered fertile periods. having sex during these times is more likely to result in pregnancy.
i got this info from a Medscape article describing home fertility testing kits.
there are kits that use other methods like saliva viewed under a microscope for a fern pattern or cervical mucous springiness.