One way you can do this is via Regex extraction. Use the "Extract Patterns" transformation, under "Column to extract from" select the column that contains the free-text field, and provide a piece of regex under "Text to Extract"
Here's a regex that you can try, I believe it should contain all of the characters that you want to keep, and pick out the illegal characters that aren't in the range of characters you supplied:
/[^\w !@#$%^&*()=+?,.:;"'<>\{\}\[\]\-\/\\]/
This means that the newly-created column will contain either one of the illegal characters in the field, or "null" if there are no illegal characters. You can do other steps like transform this column so that it contains more understandable values. Here's an example using the "Edit with formula" transformation:
While it is possible to directly create a column that says "true" or "false" to whether the free-form field contains illegal characters, I personally prefer to break this process up into two steps and use some sample data to verify that my regex is correctly picking out the illegal characters.
