In lieu of feed reporting (which is currently not available, you could consider adding the Resolved Date, Modified, and Modified By columns to a custom report. Your report probably would want to filter on Status is one of (a closed status). When you see the report results, you look for tickets that were modified after their Resolved date and whom they were modified by. You know if it is not one of your employees who modified the ticket, it was a customer sending a comment.
You can flush out the report with other needed reporting items, but I think that would get you close to what you're looking for. Please let me know if you have any any questions.