Can ascities be seen on cat scan or mri?

Yes. Ascities is fluid build up in the abdomen. It can be seen on these tests and also if big enough noted on physical exam.
Yes. Ascites, a build up of fluid in the abdominal cavity can be seen on ct, MRI or ultrasound.
Both. You can easily see it on both studies. But all you need is an ultrasound. If it's a lot, you don't need anything - just examine and feel the water in the abdomen.