How can I tell if my Bitcoin-Qt wallet is protected with encryption?
If you encrypt your wallet, it will also be encrypted for all addresses your ever create.
Bitcoin qt will ask you for every transaction you made for the password.


  Does that mean anyone can use the funds I have in my wallet on computer A, without my passcode?


At this moment yes, try to encrypt it again, maybe you made a misstake?


  The status of the transaction on computer B is also, 8 confirmations? Is this a sign of a mistake. Also the 'from' is marked as unknown - is this important?


A confirmation means, that you transaction was successful added to a block and is processed.
Some markets or platforms need a certain number of confirmations.
Your payment is normal confirmed after 1 confirmation.


  There is also no lock icon on my QT client, which I have been told there should be if the wallet has been encrypted?


Yes, there should be a "lock" icon on the right bottom. Try to encrypt your wallet again is it possible.