Checking a link
Everything else on this site explains why sources cannot answer the question. This section is the part that can.
Why the work moved to arrival
Nine source channels, and not one of them establishes key ownership. That is not a failure of curation, it is a property of what channels are. A channel carries text. Establishing that a party holds a key requires that party to demonstrate it, which can only happen at the destination.
The good news hiding in that is that it makes the job uniform. You do not need a policy for forums and a different one for directories and a third for links from friends. One check, run on arrival, works identically regardless of how you got there, which means the effort you were spending on choosing a better source is better spent here.
What the check establishes
Exactly two things, and they are narrow and absolute. That whoever produced a signature held the private key matching a specific public key. And that the signed text has not changed by a single character since. There is no partial credit and nothing probabilistic. Either the maths works or it does not.
Everything useful comes from chaining that to something you already trust, which is why the key comes first and why pinning a fingerprint is the step everything else rests on.
The four pages
How long this actually takes
Pinning a fingerprint is careful work you do once, and it takes maybe half an hour if you do it properly. Every check after that is mechanical and takes under a minute. That ratio is the reason the advice is worth following. The expensive part happens once and the repeated part is cheap.