Short answer
The best source is the organization that assigned the identifier. The code must match the exact product variant and packaging level.
Inspect packaging and labels
The digits usually appear below the barcode; preserve leading zeroes and copy the complete number.
Ask the manufacturer or supplier
Request official product data when uncertain instead of copying a similar item’s code.
Check variants and multipacks
Sizes, colors and packaging configurations may require different GTINs.
Use Verified by GS1
Verified by GS1 can help identify the organization associated with a number.
