That's a critical question when your production line is on the line! I totally get why you'd be worried - a counterfeit module could bring everything to a halt. Here's what I've found from researching this:

First, the most reliable method is to contact Rockwell Automation directly. They have a specific process for verifying product authenticity. You can call their tech support with the serial number (SN) and catalog number (CAT) from the module, and they'll check their database to confirm if it's legitimate. This is especially important for discontinued products where you might be buying from secondary markets.

Second, check the physical details carefully. Genuine AB modules have specific security features on their labels, including special printing techniques like black-on-black printing and guilloche patterns that are hard to replicate. Counterfeit modules often have barcodes that don't scan properly or have mismatched information between the barcode and human-readable text.

Third, be extremely cautious about where you're buying from. Rockwell Automation warns that any 'new' products sold on platforms like eBay or from unauthorized distributors are likely gray market or counterfeit. The only way to guarantee authenticity is to purchase from authorized Rockwell distributors or directly from Rockwell.

For discontinued modules specifically, you can also check Rockwell's Product Lifecycle page to verify the product's status and get information about its authenticity. Remember that counterfeit modules won't have valid firmware licenses and won't be covered by any warranty.

When your production depends on it, it's worth taking the extra time to verify through official channels rather than risking downtime with a fake module!