The way I do this is by converting my fmb's into fmt's (binary to text), then I can easily search through them.
My forms environment is on Solaris so after I have made any changes to the forms on my desktop I move them into Solaris and execute a script that compiles the binary and then creates the text file using the frmcmp.sh script. When I am looking for a specific item or plsql code, I can then go to the directory that contains my *fmt files and use the Unix grep command to search for that string.
FormsAPI Master is an excellent tool.
for Windows you might be able to use
FilterFilesWithUnknownExtensions DWORD value to 1 in the following registry key:
Now you can use standard Windows search.