Well, first you need to have had confirmed a pregnancy. If you have not been medically confirmed, too early in the pregnancy is almost impossible to tell even when you go to the doctor for a suspected miscarriage. Unless your HCG levels are still going crazy when you get to the ER. After 10 weeks pregnant or so, the HCG levels will probably not go back to normal right away but they do diminish greatly when you lose the fetus. There are HCG level calculators you can buy to monitor your HCG levels to tell whether your pregnancy is going normal.
However I have heard of women bleeding through their pregnancy, which lead them to believe they were having a miscarriage. It is extremely rare, but it has happened. I had spotting every month for my third son, which lead me and my OB/GYN doctor to do a bunch of ultrasounds. If you are above 12 weeks or so, an ultrasound might put your mind at ease.