A signed block

This is the only artifact in the whole collection that can settle an argument. Everything else on this site helps you form an opinion. This one produces an answer.

What it is: A block of text with a cryptographic signature wrapped around it
Where you meet it: Market pages announcing addresses, key changes, downtime
What it settles: That one specific key signed this exact text
Cannot tell you: That the signer is honest, or that the message is current

The specimen

The shape of a signed announcement. Cyan marks the four armour lines that have to be present. Lime marks the date, which lives inside the signed text and is the part most people never read. The signature itself is not shortened in real life, and the number of lines tells you nothing.

Reading it in order

Check there is a block at all. A page that announces addresses without one has not made a claim you can test. That is not a small omission, it is the whole thing.
Copy every line, from the first armour line to the last, without trimming anything and without touching the whitespace. A partial copy fails in a way that looks exactly like tampering, and people conclude the wrong thing from it constantly.
Verify against a fingerprint you already hold, never against a key the page offers you.
Read two things in the output. Which key signed, and what date is inside the text.

What the block is not

Not an encrypted message. Everyone can read a signed message. Signing and encrypting are different jobs and this is the first one.
Not a certificate. Nobody issued it and nobody can revoke it from outside.
Not fresh by default. A signature from two years ago verifies exactly as well as one from this morning, which is its own specimen.
Not proof of good behaviour. It proves authorship. Authorship and honesty are unrelated.

Why it is worth the minute

Because it is the only step in this whole subject where the answer does not depend on your judgement, your experience or how the page looks. Everything else you do here is weighing evidence. This one is arithmetic, and arithmetic does not care that the page loaded quickly or that a friend sent you the link.

What to do when there is no block at all

Treat the announcement as unclaimed. It might be perfectly genuine and there is simply no way to tell, which puts it in the same category as a message from a stranger. That is not an accusation against whoever published it. It is a statement about what you are able to establish, and the honest response is to wait for something signed rather than to act on something that cannot be checked.