you can normally tell two weeks after a missed period using a home test.
I'm irregular, so my last period was july I wasn't expecting it in August but I still took a test, and it was negative. in the beginning of September my boobs were hurting, I was really nauseous, I was hungry all the time, I slept a lot, and peed a lot, I took a test and it was negative. I took a test September 21st and it and out positive. my doctor confirmed and said I was 9 weeks. so sometimes tests are false. if its negative, do a urine test at the doctors :)