I didn't either... til the "buck" gave birth to litters a week or so apart!!
I know so much better now!
you cant always tell that they are pregnant, No.
they might "nest" (but see my other post) or become much more aggressive, eg if you go to pick them up as their time approches.
If they're outside, they might dig more, to make themselves a burrow.
they might look a little bigger, or get larger teats, but its not always easy to tell.
If/when they do kindle (give birth) do not handle the kits. They'll be bald & helpless, and easly rejected/killed/eaten by mum if she feels threatened. Or if she feels the environment isn't suitable (including if she gets too hungry or thirsty)
If she pushes them out of the nest, you're wasting your time trying to put them back, and hand rearing is difficult.
good luck,
Little Blue, and the rabbits!