V/A+ 는/은/ㄴ 걸 보면 V/A+ 는/은/ㄴ 게 틀림없다

It is attached to the stem of an action verb or descriptive verb to show that the result assumed based on what the speaker has observed will certainly be so.

이렇게 전화를 안 받는 걸 보면 또 어디서 신나게 놀고 있는 게 틀림없어.
Seeing that he doesn’t answer the phone like this, he must be having fun somewhere else.

저렇게 운동 실력이 좋은 걸 보면 아마 어릴 적에 따로 운동을 배운 게 틀림없어.
Seeing that he is such good at exercising, he must have learned to exercise when he was young.

그렇게 뭐든지 열심히 노력하는 걸 보면 저 사람은 어디에서라도 크게 성공할 게 틀림없어.
Seeing that he works hard for everything, he must be successful greatly, where ever he goes.

Conversation:

이번 선거에서 누구를 뽑을지 결정했어?
Have you decided who to vote for in this election?

아니, 아직 결정하지는 않았는데 그냥 가장 지지도가 높은 사람으로 결정하려고 해.
No, I haven’t decided yet, but I’m just trying to choose one who’s the most popular.

그 사람이 어떤 사람인지도 모르잖아.
You don’t even know who he is.

여론조사에서 높은 지지도가 나온 걸 보면 괜찮은 사람인 게 틀림없어.
He must be a good guy, given the high approval rating in the polls.

많은 사람들이 선택했다는 이유만으로 자기 의사를 결정한다는 게 일종의 군중심리 같은데.
I think making own decision just because many have chosen so is a kind of mob psychology.

정확한 정보가 없어서 결정이 힘들 때는 차라리 다수의 선택을 따르는 게 지혜일 수도 있지.
It might be wise to follow the choice of the majority when you have no accurate information to make a decision.

Related words
선거 election
지지도 approval rating
여론조사 poll
의사 intention
군중심리 mob psychology
It means the phenomenon of not having information but only following the choices made by the majority of people. It means behaving easily according to what others say and do, losing control, or getting excited easily when many people get together.
다수 majority

Source: Yonsei University. Korean Language Institute