Distinguish ajax requests from full requests in JSF custom validator

My validator needs to know if it is a full request or an ajax request. In my current solution I check the http request header for the X-Requested-With element:

public void validate(FacesContext context, UIComponent component, Object value) throws ValidatorException {
    HttpServletRequest req = (HttpServletRequest) context.getExternalContext().getRequest();
        if (req.getHeader("X-Requested-With") != null) {
           // do something
        } else {
           // do something else
        }
       ...
}

Is there a better approach to achieve this? Is my solution "safe" with respect to different browsers / javascript libs?

UPDATE:

Just found out that the X-Requested-With header is only present if the ajax request comes from the Primefaces component library (the <p:ajax>tag).

It is not present if I use plain JSF <f:ajax>. So my approach won't work with <f:ajax>.

Using <f:ajax> there is a different header:

Faces-Request:partial/ajax

The solution proposed by Osw works for <f:ajax> and <p:ajax>:

PartialViewContext#isAjaxRequest()