In the post I recommended to sign a test message with the private key and check it with the public key - which has the same effect, it's just the other way round. Whether that's "trivial" is another question, given how arcane the usage of the tools doing these things is.

There are multiple ways to properly check this. However the point is: Symantec didn't do it and the vast majority of the guides you find on the Internet about this are wrong.