If there is a longer fork with more work that your node thinks is invalid, you'll get this alert:
Warning: Displayed transactions may not be correct! You may need to upgrade, or other nodes may need to upgrade.
(see the GetWarnings() function in main.cpp).
If you are on the fork with the most work... I suppose if the second-best chain was forked more than 6 blocks back and contained more than... oh, 5 blocks and the timestamp on the last block in that chain was less than ?an hour ago? that could trigger another alert.
All that might be tricky to implement-- either (or both) forks might themselves have forks. Or, theoretically, there could be three or more active forks, some of which might have equal proof-of-work...