One thing to note is that sql server defaults to a clustered primary key. If the tables have a Primary key they most likely (Note: not always and certainly could have been created with a nonclustered primary key) are clustered on that key if they have no other indexes. i know when im creating tables i just declare a primary key and let sql server use it as its clustering key unless i know what data will be going into the table and that a different column will make a better clustering key.

EDIT: After posting and then looking at sys.indexes after creating a test table the primary key index is shown as clustered. so depends on where you looked as to the validity of my comment to the OP's situation.