How to check if git merge problems have been fixed?

Do:

git diff -S "<<<<<<< HEAD" -S "=======" -S ">>>>>>> $(git name-rev --name-only MERGE_HEAD)" HEAD

This compares the contents of the worktree with HEAD, but only shows any output if one or more of the three types of merge marks is included in the change.

For example, if you are merging from a branch called develop, an unmerged file might look like this:

public void fooTheBar(Bar input) {
<<<<<<< HEAD
  if (input == null) {
    throw new Exception("input cannot be null!");
  }
=======
  Console.WriteLine("preparing to foo the bar");
>>>>>>> develop
  input.foo();
}

So to ensure all files have been merged, you need to search for any of the following three lines:

<<<<<<< HEAD
=======
>>>>>>> develop

And that is what the -S arguments in the command do. Since it won't always be develop, we use the command:

git name-rev --name-only MERGE_HEAD

to get the name of the branch that you are merging into your current branch.

(You could probably search for just one of those lines, but searching for all three is more robust, and will show you cases where e.g. you forgot to remove just one of the lines.)

Since the command compares the worktree to HEAD, and not just to the staged changes, this will work even if you have git added a file that still contains conflicts.