If I try this process using a private key of a Casascius token that has already been redeemed / spent, I should see the transaction attempt to occur in the wallet but not get confirmed (obviously because its already been redeemed).
If you enter a private key which has already been redeemed, the balance of the wallet should be zero. You should also see an outgoing transaction (a transaction with negative amount) spending the fund in "History" tab.
The outgoing transaction has definitely been confirmed.