How do I know I am able to conceive?

I have been wanting to have a baby. My fianc and I usually have sexual intercourse two to three times a week (it is only that much because he is away at college the rest of the days). It has been almost a year, and no baby. Are we not having sex enough? I heard sperm can live in a body for three to five days. Is this true? Is there any tips on how to conceive? And how do I know if I am able to conceive?