I would say "I think so" in polite way, そう思います sou omoi masu / そうだと思います sou dato omoi masu or just そうだと思う sou dato omou. "I guess (so)" politely say - そうだと思います sou dato omoi masu or informally say, 多分 tabun :maybe. I hope it helps you.

思います 私もそう思います　I think so too. XXXXXX と　思います (I think) XXXXXX と　思っています (I have been thinking)