I think your best bet would probably be a coverage tool. There're plenty for both *nix and windows. If you have unit tests, it's easy - if you have a low test coverage, then the uncovered code is either dead or not tested yet (you want both pieces of this info anyway). If you don't have unit tests, build your app with instrumentation provided by one of those tools, run it through some (should be all ideally) execution paths, and see the report. You get the same info as with unit tests, it will only require a lot more work.
Since you're using VisualStudio, I could provide you couple of links which you could consider using:
- Coverage meter
- BullseyeCoverage
Neither of them is free, not even cheap, but the outcome is usually worth it.
On *nix-like platforms gcov coupled with tools like zcov or lcov is a really great choice.
