formal
そうだと思ってました。
informal
そうだと思った。 or やっぱり(ね)
やっぱり is a spoken language, and a very casual phrase.
やっぱり means "That’s what I thought" "I thought so" "I knew it."
I think you can use やっぱり when you talk with your friends and someone close to you.
Some young Japanese say やっぱ(ね), and some Japanese say やっぱし(ね).