Hi Melissa and welcome,
As far as how do we know yet, well Caroline and I are both a little "odd" in that we both tested
at only 9 dpo. That said, technically the FRER tests are supposed to pick up 4 or 5 days prior to your period being being late (First Response Early Result) If you are having symptoms, you likely have already implanted and therefore may test positive.
There is also a Trying to Conceive board (I think under fertility) and in that there is a 2 week wait thread (the 2 weeks between ovulation and period due) That is where everyone keeps track of who is testing and what their results are. (lots of fun over there, Caroline and I still hang out there!)
If your period is due this weekend but your edd is Sept 11, I assume your cycle is longer than 28 days?? I have a pretty reg 28 day cycle and my period was due Jan 3, my edd is Sept 13, 2004. How long is your luteal phase normally?
Well, hoping your suspicion is right and you will be hanging out here for the next 9 months!!!