You may be able to see a distinct line from the middle front of her upper back legs curving down to her belly about a third of the way along her teat line from the back. The vulva may start to enlarge and look softer around three weeks before farrowing. In a gilt the teats may enlarge slightly or may look more or less the same. Nearer the time you may be able to see the piglets moving if she's lying on her side. You note I use the word "may" rather a lot. Her appearance will, of course, depend very much on how many she's carrying - two will be barely discernible, twelve rather more so.