Chilkat’s Online XML Signature Validatorhttp://tools.chilkat.io/xmlDsigVerify.cshtml

gob.es FacturaE online validator: http://sedeaplicaciones2.minetur.gob.es/FacturaE/

ETSI Signature Conformance Checker (requires credentials)  http://signatures-conformance-checker.etsi.org

Validate UBL:  https://www.moj-eracun.hr/exchange/validateubl

UBL 2.1 e-Invoice Validation: https://joinup.ec.europa.eu/solution/ubl-21-e-invoice-validation-service/about

FatturaPA (fatturapa.gov.it)  http://sdi.fatturapa.gov.it/SdI2FatturaPAWeb/AccediAlServizioAction.do?pagina=controlla_fattura

ADSS Signing Server: https://account.ascertia.com/demos/xmlsignatureverificationstep1

XmlValidation.com:  https://www.xmlvalidation.com/ I don’t trust this validator. Change the signed XML document to deliberately introduce errors and then check to see if the validator returns “No errors”. My testing, as of 6-Nov-2018, shows that signed XML w/ modifications still report “No Errors”.