You can do a boolean query using a case expression.
As of JPA 2.0 (Java EE 6) you can create a TypedQuery .
String query = "select case when (count(*) > 0) then true else false end from ......"
TypedQuery<Boolean> booleanQuery = entityManager.createQuery(query, Boolean.class);
boolean exists = booleanQuery.getSingleResult();
In JPA 1.0 (Java EE 5) you must use an untyped query.
Query booleanQuery = entityManager.createQuery(query);
boolean exists = (Boolean) booleanQuery.getSingleResult();