You can see which forms that fields are used in by going to the custom forms section of Setup and clicking on the "Fields" tab. If there is no Forms column, you can add one via a custom view and selecting parameter>>forms.
The way I've found to see which forms are being used is to do a report for each object type. For instance, A Project report that is grouped by category (forms are called categories in the API) shows me which forms are being used and how many times. It does not show forms that are NOT being used. You'll have to compare this to a full list of forms to see which forms are NOT present. It's a bit backwards but it works.
This method can apply to user forms, issue forms, etc.
