것 같다 vs 나 보다 vs (으)려나 보다
* ~ 것 같다 => "It appears/seems that ..." - general expression which can be used for any tense (ㄴ 것, ~는 것, ~ㄹ 것).
* ~나 보다, ~ㄴ가 보다 => almost the same as ~ 것 같다, but more suitable for a perception than a thoughtful guess.
* ~(으)려나 보다, ~(으)려는가 보다 => "It appears something is going to ..." - works only for the (near) future tense.
So ~것 같다 is the most general and can be used for inferences, while ~나 보다 and ~ㄴ가 보다 are more for perceptions.
~(으)려나 is short for ~(으)려 하나 which means "is about to do/cause something" (implying an intention or something about to happen), and therefore is a special case of ~나 보다.
* 그 일은 내 잘못이었던 것 같다 = I seems I was wrong in that matter - past tense.
* 그 일은 내 잘못이었나 보다 [잘못이었는가 보다] = (same - ~(으)려나 can't be used)) - past.
* 난 이게 끝인 것 같다 = 난 이게 끝이나 [끝인가] 보다 = It appears this is the end for me - present.
* 경기가 나빠지려는[나빠질] 것 같다 = 경기가 나빠지려나 [나빠지려는가] 보다 = It appears the economy will get worse - (near) future.
( ~지려는 indicates a transition (present to near future), like "is about to", while ~질 indicates a firmer future)