Use the can.map.validations plugin. Then in each model class's init() method you can declaratively set the validations with this.validate(), this.validatePresenceOf(), this.validateFormatOf(), etc.
What we do is to set up a compute around each instance's errors() method (which is provided by the validations plugin), and call it computed_errors. Then you can simply do:
- {{#if computed_errors.<name of some validated attr>}}<label class="error">Error message</label>{{/if}}
in your mustache.
Note that errors() returns null if there are no validation errors, so it's very useful for disabling the submit button in your form.