If you are naming your foreign key fields consistently for example extras.products_id -> products.id or using some fairly unique data type like GUID for your keys it could be intelligently inferred.
If you’re allowed to query the data you could use the column name/data type to come up with lots of good candidates, and then if then verify by checking whether the actual values correspond (assuming that it’s something like a uuid or username column and not a serial/int column)
Column names could sound like foreign keys on pure chance and the data type doesn't show which other table it's referring to.