By ultrasound, there is no other way. It is too early for a normal checkup to tell and HCG blood test to reveal exactly how far along you are. Do a home pregnancy test, or get a blood test and wait until you're further into the pregnancy to find out how far along you are.
If you don't know when you conceived (you'd need to know when exactly your ovulation occurred), add seven days to the first day of the last menstrual period; that is when your pregnancy can be said to have begun. If 5 weeks after that date a pregnancy test comes up positive, you're probably 5 weeks pregnant. Consult a gynecologist for a more accurate result.