Could very well be. You can try and check all Received:-Headers, but there's no obligation that the MTA actually has to write whether TLS was used, nor can you

Could very well be. You can try and check all Received:-Headers, but there's no obligation that the MTA actually has to write whether TLS was used, nor can you be sure that the MTAs aren't simply adding some TLS string without actually having used TLS. Of the big email providers, only Gmail supports TLS.