Bitcoin Core will tell you the size of the keypool through an RPC command, which you can also type in the debug->console window. Here you can see the result of me setting the default higher:


>getwalletinfo

{
"walletversion" : 60000,
"balance" : xx.xxxxxxxx,
"txcount" : xxx,
"keypoololdest" : 1308965703,
"keypoolsize" : 201
}


To have Bitcoin Core maintain a larger keypool size to future-proof wallet backups, you can increase the keypool size as an option in your Bitcoin configuration file. I have written a guide with an example configuration file which includes the keypool option - simply remove the comment "#" and change the setting from the default of 100. The "Bitcoin data directory" link there also takes you to documentation to locate your OS data directory.

If you want to use up the keypool, you can issue the "getrawchangeaddress" command as many times as you have addresses. Encrypting a wallet will also discard the keypool and generate a new one.