[tor-talk] Identifying use of relay as an Entry node

[grarpamp]

When constructing my own path (as a client) through a relay
(running as a separate process on a separate network), I
obviously know from the client controller when said relay
is being used as an entry node.

How can I tell when said relay (to which I have full access)
is being used as an entry node by other clients on the net
(those not under my control) ?

Or put another way, were said relay to be instrumented,
would such identification be possible?