Assuming that medical notes would be set up for the participant at the point they consent for the study, then you could train the site staff to retrospectively document the consent process at that time point, and make it clear that it was written retrospectivey and why. You could also provide a screening checklist to be completed by the site staff to capture details of the consent process - i.e. date info sheet provided, what version was provided, any discussions regarding the study (in person or by telephone), date that consent was signed, version of consent form used. The screening checklist (signed and dated by site staff) would then be the Monitor's source data for the consent process.