You can run bitcoin -printblocktree then look in debug.log to see any chain splits your client observed. My copy has seen three throughout its lifetime, all of them only a single block long.
It seems low. All invalid blocks are a single block splits. Whenever such a block is deemed invalid by the bitcoin program (because an alternative forked tree grew longer) and transactions are merged into the current tree, bitcoin prints "REORGANIZE" into debug.log. I grepped 4 such instances in April only. There should have been been more because my deamon may have picked the "correct" tree and didn't need to reorganize anything.