First, make sure you can trust the seller. Burberry says it doesn't authenticate things for customers and that the only proof of authenticity is a receipt from Burberry, Burberry.com, or an authorized retailer.
Ask the seller for a copy of the receipt or order proof if you didn't buy from one of these places.
You can find the exact model on Burberry's website by cut, lining, color, and writing on the hardware. Some trench coats are currently "Made in Italy," so that fact alone isn't a red flag. Look closely at the product pictures, buttons with logos, and inner details.
Look for a clear style ID and a 13-digit serial number on the inner care/brand label. Bad signs include writing that isn't neat or codes that are missing. Check the serial number with reliable sources.
Check for signs of quality such as clear "BURBERRY" engravings on heavier buttons, accurate stitching, and straight check placement near the neck tag. Often, inconsistencies here mean it's a fake.
These are key methods that we can offer. If you have more, please share.