Have a look at Hibernate Validator, which allows entity validation (using annotations).
http://www.hibernate.org/subprojects/validator.html
In short, you annotate your field constraints by placing hibernate validator/ JPA annotations above them. (E.g. @Min(10)) and use the following piece of code to find any invalid fields;
ValidatorFactory factory = Validation.byDefaultProvider().configure().traversableResolver(new CustomTraversableResolver() ).buildValidatorFactory();
Validator validator = factory.getValidator();
Set<ConstraintViolation<BaseValidationObject>> constraintViolations = Validator.validate(myEntityToValidate);
If you need to validate specific relationships between entities, you can write custom validators to fit that need.