You can query the rather misleadingly named [WebLink][1] SObject to find all of the information you need about a custom button:

    Select Width, Url, SystemModstamp, ShowsStatus, ShowsLocation, ScontrolId, 
    RequireRowSelection, Position, PageOrSobjectType, OpenType, NamespacePrefix, Name,
    MasterLabel, LinkType, LastModifiedDate, LastModifiedById, IsResizable, 
    IsProtected, Id, Height, HasToolbar, HasScrollbars, HasMenubar, EncodingKey, 
    DisplayType, Description, CreatedDate, CreatedById 
    From WebLink
    Where PageOrSObjectType = 'ObjectOfInterest__c';



  [1]: http://www.salesforce.com/us/developer/docs/api_meta/Content/meta_weblink.htm