This might work:
* Disconnect from the internet, or act before a new block is produced.
* Subtract your target block number from the last block you have. If you know a specific past transaction of yours was the last one before the fork, you can just hover over it to see the number of confirmations.
* In the debug console, use the above number (N) as parameter for:
getbalance *
Just so I am sure I understand correctly, there is no ONE address for a bitcoin core wallet, right? It is all of the receiving addresses we are talking about? Or is there one address?
Normally it's multiple addresses, unless you've repeatedly used a single one, including for transactions change.