I ran across some instances in a legacy database where some numeric codes where being stored with no reference table for these values. Luckily, the values were enumerated in the application code. Obviously it is preferable to have these values in the database where a foreign key can restrict the values stored in the referencing column, as well as give a way to interpret the values.
I created a script to search for columns in a database that don’t have a foreign key referencing them. I include a parameter to limit the search to numeric values. Other than a generated primary key value, we would imagine that most numeric values are referring to another value.

The script is available on GitHub.