Verifying Oracle*Net network encryption
I too am looking for a better way to do this, but what I've found is that you can enable tracing on the client side and then search the tracefile that was created.

I have this in my client-side sqlnet.ora file:

DIAG_ADR_ENABLED=off
TRACE_DIRECTORY_CLIENT=/tmp
TRACE_FILE_CLIENT=nettrace
TRACE_LEVEL_CLIENT=16


If you're running the client from the same server, you can override TNS_ADMIN to point to a different directory (you may need to copy tnsnames.ora, etc, to that directory as well).