You can do as colorit2 has suggest but you can also look at the version file in the main package you get from themeforest for all the changed files.
If you don't have an accurate changelog of the changes you have made along the way, you can also work on diagnosing any individual bug you have one on one with a developer. It isn't ideal as you've already purchased the theme and Kriesi has probably fixed the bug.
The best solution in your situation is comparing the files you have changed with those of the updates. If you don't know which, the a full on comparison with all of the files listed in the version since your version is the next best thing.