You can check using google, other search engines and some websites. If you copy a couple of lines from the answer, paste it on google and search, if its plagiarized you will find atleast the sources used or the whole article it was taken from.

If the answer was elaborated on a bit and had no literal word by word similarites than you can take the answer as an unique one. The answer who wrote it added value to it as he saw fit. Because not all topics on musing can be known to everyone.

But if the answer was just a copy paste a simple search would show even the slightest bits of the original one it was copied from.