Bribery is a particularly difficult fraud to prevent or detect.
Charles Hall has some tips at CPA-Scribo on How to Detect and Prevent Bribes.
His article describes that such frauds are typically found by a tip. He provides some ideas for tests auditors could run in search for telltale patterns.
The post also has creative ideas a government agency, or perhaps private company, could take to work against fraudsters taking bribes.
If you are an auditor, especially of a government agency, check out the post.