Form Grep is the solution
I also found very useful Form Diff, a software which allows you to locate differences between 2 Forms.
A cheaper solution to find some text into a form (*.fmb *.fmx), library (*.pll *.plx) or report (*.rdf *.rep).
Under Linux simply type:
find . | xargs grep 'string' -sl
The -s is for summary and won't display warning messages such as grep: ./directory-name: Is a directory
The -l is for list, so we get just the filename and not all instances of the match displayed in the results.