They get an "idea" from your fundal height. I had excess amniotic fluid with DD, and my fundal height was measuring about 4 weeks ahead, she did a scan and a series of measurements and determined that I had quite a bit of excess fluid, she then did testing for GD and some BT's to see if there was an infection as a cause, in the end everything came back negative. This pregnancy I have low amniotic fluid, my OB picked it up on a scan 4 weeks ago, (I am having fortnightly scans checking my cervix length), but she seemed less concerned at my last scan as there is little fluid around bub, there are pockets at either end.

HTH's