It's only possible for a resolver to check the last in the chain of DNS forwarders to see if DoH transport is being used, so the reliability of such a check (like
https://1.1.1.1/help) is limited and only the external IP address (which is not reported in the
1.1.1.1/help result) can be reported with any certainty (and with NAT, even an external IP address isn't enough to truly identify the client). If the DoH (or DNS over TLS) client uses a client certificate, showing that would be a more meaningful check, but it would be hard for DoH services where the TLS termination is handled separately from the DNS resolution to see the client certificate.