Makay, the signs are really quite simple. Yes, sows will drink more water if they are pregnant. Further, they will start to fatten up on the sides. You should be able to tell easily if they are pregnant or not. And sometimes if you look closely, you can see their babies kick. Also, pick up your sow very gently and fill around her belly. You should be able to tell if she's pregnant or not. It's very obvious once the signs really start to show.
And yes, don't forget to check with your vet and make sure he/she is very literate with Guinea Pigs.
All the best luck to you!
