It's very similar to "I suppose", somewhat more careful ~것이 아닌가 싶다: "I suppose" ~것이 아닐까 싶다: subjunctive of above. = "I'd say..." note that the subsentence is transformed to the pseudo-noun '것', and it's suited for past and present. For the future tense however, ~지 않을까 싶다 is the right variation. example: 지금 바로 출발하면 두 시간 이내로 목적지에 도착하지 않을까 싶습니다. I'd say we will arrive at the destination within two hours, if we depart right now.

-(으)까 싶다 : 마음속에 앞말이 뜻하는 행동을 할 의도를 가지고 있음을 나타내는 말. -(으)ㄹ까 싶다: one wonders whether it might be the case that... The tentative question in -(으)ㄹ까 functions as a quoted thought. 어쩌면 그가 내 꿈을 들여다보는 것은 아닐까 싶을 때도 있다. There were also times when I wondered whether perhaps he wasn't peering into my dreams. You asked similar question 3 years ago. https://www.italki.com/question/190798