There are a few different ways to say it. "I bet your English is really good!" 1. 너 틀림없이 영어(를) 아주/정말 잘할 것 같아. (informal) 2. 틀림없이 영어(를) 아주/정말 잘하실 것 같아요. (formal) #2 has no subject, but the honorific suffix 시 in 잘하실 (잘하시(다) + ㄹ) indirectly indicates the subject is the other person, because you never use an honorific form for yourself. This is one way to avoid saying "you", like when you don't know the person's name. In place of 틀림없이(surely), you can also use 보나마나(no need to even see/check) or 볼 것도 없이.

나는 당신의 영어는 정말 좋은 내기!