The exists query is a convenience that sets the query to only select the Id property, use a maxRows = 1 and tests if the query returns a row.

boolean userExists =
    new QContact()
       .email.equalTo("rob@foo.com")
       .exists();

select t0.id from contact t0 where t0.email = ?  limit 1

We could instead write this query using findIds() and maxRows() but it is a common enough pattern to perform checks like this that we have this exists query.