I guess it's not as though he doesn't look like an athlete.
If you didn't know better you would think she was an actress.
この場合、次のように言うのが良いかなと思います。
ーI guess it's not as though he doesn't look like an athlete.
「彼はスポーツ選手に見えないわけでもない」
ーIf you didn't know better you would think she was an actress.
「そうでないことを知らなかったら、あなたは彼女が女優だと思うだろう」
ご参考まで！
カナダ 
