Put your search terms into the iDiscover search bar as you would for any other search. When iDiscover completes your search and you are presented with a list of results, you can see which items belong to Electronic Legal Deposit as they contain this red text: 

You can filter your results so that you only have Electronic Legal Deposit items in your results. On the right-hand side of the screen scroll until you find ‘Collection’ in the list of filters. 

If you click this box your results will be filtered to only show Electronic Legal Deposit items:

Or if you click this box your results will be filtered to exclude Electronic Legal Deposit items: