It can be useful to mark each complete reference in the list in a highly visible way whenever you discover, check and correct an in-text reference to that source. Adding an asterisk or other identifiable mark at the beginning of each complete reference in the list will achieve this, and using red or another bright colour to do so will make each mark stand out effectively. Once you have made your way through the entire paper, these marks will show you which sources in your list have been cited in the main paper. If any do not bear marks, do a second search through your document for them. If they are not cited in the paper, they should probably be removed from the list, but if they are, they too should be checked and, if necessary, corrected before they are similarly marked in the list.