There are two places to get some free information about what is and is not negotiable. The fastest is probably OPM’s web site and particularly its Negotiability Determinations guide. The guide is organized by topic and reduces cases related to the topic to two or three line summaries. The real value here is the code used at the end of each summary. OPM marks a case as either N or NN. The former means the proposal was negotiable and the latter means non-negotiable. If you are looking for some quick ideas of what you can propose in relation to a change, this is the quickest.
The other source is FLRA’s web site. If the OPM guide gave you a case citation, just plug it into the case search engine which you find by clicking the “Decisions” menu on the home page black bar and then “Authority Decisions” from the drop down menu. If you do all that it should bring you to this page, where you can search by case citation or even by keyword.