Rely on how your body feels. I could tell when I was pregnant because I felt different. A blood test is more reliable than the urine test, but if you're this far into it (sounds like you are 2-3 months along from your comments above), the urine test should be accurate. Start taking those prenatal vitamins if you haven't already :-) .
You may not have a false positive, but it's possible that you can have a positive pregnancy test but the pregnancy doesn't stick.