Did you look at her teats? Is she developing an udder? Because of the rumen, you can't look at how "fat" a goat is... the rumen can get quite large especially if they've been out on fresh forage. An easy way is to look at the teats. Any development there would indicate pregnancy. But, @moomoodiddy is quite right - you can send in a blood sample and they can tell you if she's preggers or not. That's the best and most positive way to tell.