>A) which of the keys was used to sign this block
? You know this, otherwise signature can not be validated.
> B) which key should have been in use at that time
That's true because of the key's construction.
Suppose that you have the initial key (key_0) constructed on 24 Aug 2012, 00:00 UTC. You encode the key update interval (say, 10 minutes) in this key and start using it. Every 10 minutes you then generate the next key and use it for signing.
Each signed block is numbered, without gaps.
If it turns out that your current system time is ahead of the key's time (because you've resumed your computer from a long sleep) then you insert a special "skip-time" block, signed by your existing (and now obsolete) key and the time-derived key. I.e. that block in essence authenticates a legitimate gap in the log history.
Again, I'm making it up as I go along :) So I might be incorrect.