The elements, which are content items, are references in content areas. So, you need to find all pages (and blocks, a form is usually a block.) that has a content area property (stored in the LongString column) that accept form elements, then you need to parse the values of those properties. A content area property has a list of content GUIDs. This is probably nothing you want to do directly in the database. I would recommend using the API instead. You can use IContentSoftLinkRepository to do lookups where content is being referenced.

Thanks for the intel. 

I had all the ID's, so I used this url to find all items in forms:

mysite.com/EPiServer/CMS/#context=epi.cms.contentdata:///28275&viewsetting=viewlanguage:///en

where ID 28275 was one of those I had. If the content item is used somewhere, this page Optimizely shows that with a small top banner saying:

"Changes made here will affect at least 1 item:"

That links to the form it is inserted at, and going to the form shows a similar top banner where the same link goes to the page, where the path is shown below view and preview buttons.
Took a while to finde them all, but jobs done.