What is another way to find out how long i've been pregnant besides counting from your last menstral cycle?

i took a pregnancy test at the very end of july and it came back negative. near the end of august, i had an incident where the condom broke and i took EC. i didn't start my menstral cycle which means i could have already been pregnant. in late september/ early october i tooka nother test that came out positive. i don't know how long i've been pregnant and don't know how to find out. help!