— Did you have anything to do with the things that were put on the wall there?
— No, that would’ve been Sam.
I’ve always seen would’ve been in conditional sentences, as in "If you were smarter, you would’ve bought it". But the usage in my example isn’t part of a conditional sentence, is it?
Can’t find an explanation regarding that usage. But I reckon it’s something along the lines of "must/might have been". Is that usage informal?
It is a conditional, in an idiomatic use.
The implication is "I didn’t who did this, but I can work it out, and if my reasoning is correct, it would have been Sam"
The perfect modal is here indicating tense: the things were put on the wall in the past.
A similar idea could also be expressed with "It must have been Sam". "It would have been Sam" expresses rather more confidence in the deduction. "It must have been Sam"="I can’t think of anybody else." /"It would have been Sam"="I can prove it"
It’s conversational, but not very informal.