How can I monitor a production database to see if there are table scans happening?

How can I monitor a production database to see if there are table scans happening? And what indexes might fix them... I'm told that we have adequate indexes but I can't see every code path, and we have too many people touching the codebase (hey, lots of developers can sometimes be a good thing) so what's the most comprehensive way to see how if we've missed indexes based on our production load? SQL Server 2008 R2, C#/ASP.NET code, NHibernate are the key factors in use.