Run the vin decoder and look for option:SA 5GA Alarm pre-wiring
The old thread linked in the first post also says:
Compatibility: Cars that are pre-wired for the alarm system (option code SA 5GA). These particular kits are only compatible for E90/1/2/3 up to the production date of 03/09. Note a production date before 03/09 doesn't necessarily mean you're pre-wired either as pointed out by Rampage9. I think if your rear view mirror has the "rudolph nose" then it's a good sign you're pre-wired but you should follow one of the DIY's to verify the wiring as well.