First, when a error ocurrs when the component are validanting or converting the request value in the phase of Process Validations the jsf-impl throws a ConverterException or ValidatorException that excepction is catch in the component that add a FacesMessage with the severity set a FacesMessage.SEVERITY_ERROR, for that you can't use the severity property to identify the cause of the FacesMessage as suggested by Victor Hugo.
All Standar Validation message errors start with Validation Error this can serve to help identify a FacesMessage for a validation error
Since JSF 1.2 you can use in the components that implements the EditableValueHolder interface as h:inputText the converterMessage and validatorMessage this you can use to show a message when a conversion or validation ocurrs.
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Ok, wandroide I think I understand you, but here you have a problem the FacesMessage is designed to be a general class, since any developer can use this class to show any message (regardless of whether validation error messages or conversion) in the redered page whith the help of <h:message> or <h:messages>, and for that reason FacesMessage don't have properties to indicate the reason of the message.
As I was saying in the previous post the most standar validation message errors contians the word Validation Error, except for RegexValidator that contains Regex Patter or BeanValidator, you can use this to identify if the message was produced for a ValidatorException.