Pregnancy Tests detect the presence of the hCG hormone in your urine.
 
Depending on the sensitivity of the test, many tests can show a positive within 7-10 days past ovulation. 
 
Whether you are using a mid-stream test or one that you dip into a cup, if you are pregnant a line will appear within 2 to 3 minutes.  Even a faint line could mean that you are pregnant.
 
However if you get a false test, it doesn't automatically mean that you are not pregnant.  If you still haven't got your period within 3 or 4 days you should consider testing again.