The best way to know if you are pregnant with twins is to have your doctor perform an ultrasound. Once the fetus(s) are visible on an ultra-sound the doctor will know 100% if you are having twins or not. If the ultrasound does not return any results(this can occur during the first several weeks), the doctor will often draw blood and test HCG levels, as well as other hormones in the blood. Blood test can only provide so much data, and are not always 100% accurate when it comes to knowing about twins. It is best to wait until the egg has reached fetus stage and when two are visible on the ultrasound, you'll know 100% for sure.