Usually they stipulate that they are to be updated every X days/weeks/months/whatever. So if you see one that promises to update every Monday, for instance, and it's old, that would be the same as being abandoned.
Nothing, at the end of the day, whether giving someone else your data is a good idea or not remains a question of trust.
Do you trust them to stop updating the canary in case they become compromised?
If the answer is no, then you probably shouldn't be giving them valuable data.
In my mind, the "correct" way to do this is to:
1) pgp sign the entire warrant canary statement with a published key.
2) add unfakeable news headlines to the canary itself (such as recent stock prices or sports scores)
3) date the warrant canary and include a well defined schedule of updates
I believe this is the authoritative example:
But a signed message that's publicly audited as authentic and stored non-centrally seems perfect for canaries....
If they disappear, that indicates something. If they stay there without change, that indicates something too. Or am I missing something?