OK, so I'm ttc no 2 and this is driving me nuts.
Every month I analyse all the sporadic 'symptoms' I have, nausea, tiredness, weeing a lot (all the norm for me anyway really) and I think 'Yes, this time its happening' and then it's not.
Also, I feel phantom kicks all the time (and I have been pregnant before, so I know what they're like) and wonder if perhaps I'm one of those women who's 38 weeks and hasn't realized 
But still, you have to wait until your period is due. Or worry that your period is actually implantation bleeding, to the point that you wipe and then compare it with Google images of implantation bleeding.
Or worry that I'm one of those women who has periods all the way through - see above.
You can always take one of those tests that are said to work days early but then whatever the result you doubt it!
Surely by now, the year when Michael J Fox went 'Back to the Future', we should have this nailed?