Grammar
-(으)ㄴ/는 걸 보니까
judging from the fact that
Form
Verb + -는 걸 보니까; past verb or adjective + -(으)ㄴ 걸 보니까 — “judging by”, “seeing that”.
Function
Draws an inference from observed evidence. It contains a nominalized clause and 보다 followed by -(으)니까.
Why this form?
-(으)니까 — “since / because”: directly gives a reason without necessarily referring to observed evidence.
Forms not to confuse
-(으)니까
비가 오니까 집에 있어요. — Since it is raining, I am staying home.
Examples
아까 질문하신 걸 보니까, 서로를 따로따로 찾으신 것 같네요. — Judging from your earlier question, it seems you found each other separately.
표정을 보니까 기분이 좋아 보여요. — Judging by your expression, you seem happy.