Lots of libraries are capable to read, edit or manipulate PDF files. I have been using iText for filling in an editable form in a PDF file.
This library is pretty simple to use, the following snippet will let you know the names of the fields that are editable in a PDF file.
Put this snippet in a main and you can test it ! I used iText 5.5.3 in this example.
You can display other information about each form (see Java classes Item and AcroFields), for example you can display the type of each field : input text, checkbox, radio button, etc.