SELECT DISTINCT D.SiteID, D.WebId, W.FullURL as WebURL, D.Id As DocumentId, 
                D.DirName, D.LeafName, tp_ID As WebPartSK 
FROM       dbo.Docs D WITH (nolock)  
INNER JOIN dbo.Webs W WITH (nolock) ON D.WebID = W.Id 
INNER JOIN dbo.WebParts WP WITH (nolock) ON D.Id = WP.tp_PageUrlID 
WHERE WP.tp_ListId Is Null AND WP.tp_Type Is Null AND WP.tp_Flags Is Null 
      AND WP.tp_BaseViewID Is Null AND WP.tp_DisplayName Is Null  
      AND WP.tp_Version Is Null 
AND WP.tp_WebPartTypeId='<your web parts id>' 

Source for this query is Ryan's comment her:

http://stackoverflow.com/questions/1498409/sharepoint-find-where-webpart-is-in-use