Oh man, I feel your pain! When your production line is dead and everyone's claiming to have that last S7-400 CPU, it's a nightmare. Here's what I'd do to spot the fakes from the real deal:

First, contact Siemens technical support directly with the serial number. According to Siemens' own forums, if you have any doubts, just email technical support with the serial numbers and they'll verify authenticity for you. This is your best first step.

Check the physical details carefully: Look for consistent serial numbers, holographic labels, and proper anti-counterfeit features. Genuine Siemens parts have specific packaging and labeling that counterfeiters often mess up. Pay attention to the quality of printing, materials, and overall finish.

Watch for impossible serial numbers - if you see multiple "new" parts with identical serial sequences, that's a huge red flag. Each genuine part should have a unique serial number.

Test the firmware if possible. Counterfeit parts can't perfectly replicate Siemens' firmware. When you install it, if the PLC flags it as an unsupported device or shows weird behavior, that's a giveaway.

Ask about warranty. Legitimate suppliers of genuine parts usually offer 12-24 month warranties. Counterfeiters won't because their parts fail quickly. Also, check if the supplier specializes in hard-to-find Siemens parts - companies like IDE Electronics International focus specifically on obsolete Siemens components.

Remember, S7-400 CPUs are being phased out, with discontinuation milestones happening through 2025-2026. If someone claims to have unlimited stock of "new" discontinued parts, be very suspicious.

When in doubt, go with reputable suppliers who specialize in legacy Siemens parts and can provide authentication documentation. It's better to wait a bit longer for a verified part than to risk your entire production line with a counterfeit that could cause system-wide damage.