The find function and the ~= operator are excellent for searching text.

The find function will search for a string of text and return the position of the text if found.  This can be used in conjunction with an IF statement that says if(find([Remarks], "past")>0, "Past Due", null).

Users may employ "~=" in an if statement in order to see if a String column contains a particular string.  

if([Remarks]~="past", "Past Due", null)

Note, these methods are case sensitive.