The way I do it is to use the standard Windows Search function on my project directory. Choose to search *.prg, *.sct (for forms), then fill in the "containing text" field with what you're searching for. It gives you a list of all files that contain what you put in.
If you want to figure out exactly which object in your form contains the procedure you're looking for, just open the *.sct file in a text editor (I use EDIT, myself) and search for a string. You should be able to identify the object by the information found above the cursor position. Just make sure you do NOT save the file, even if you don't alter it. You'll never load it again.