On SQL Server, this will list all the indexes for a specified table:
select * from sys.indexes
where object_id = (select object_id from sys.objects where name = 'MYTABLE')
This query will list all tables without an index:
SELECT name
FROM sys.tables
WHERE OBJECTPROPERTY(object_id,'IsIndexed') = 0
And this is an interesting MSDN FAQ on a related subject:
Querying the SQL Server System Catalog FAQ
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