Sounds reasonable to me to look e.g. for a “BEGIN: VCALENDAR” and a “BEGIN: EVENT” as well as for a closing “END:VEVENT” and “END:VCALENDAR” as a basic validation. I think from a user perspective this should be fine, the subscription would be fast and as it can happen at any time that a once valid resource becomes invalid (maybe due to an error while the .ics file was created, or a failed file download, or…) I don’t see a drawback in validating the whole file only when synchronizing.