queriing the version of libpq

libpq BEFORE 9.0 are not able to decode that encoding. 

I am programming with Python, using psycopg2. When psycopg2 is linked with libpq 9.0, everything works fine with hex-encoding; if psycopg2 is linked with libpq < 9.0, decoding hex-encoded bytea fails (it stays hexencoded).

This happens because in default configuration psycopg2 calls the libpq-decode-encoded-bytea function (which is the way it should be done).  

Now I would love to have an additional check "is the used psycopg2 linked to an advanced-enough libpq", to be able to set bytea_output to 'escape' if the libpq is not worthy.

My question: Which way is available to query the linked libpq version? 

My other option is to select 'something_that_gets_casted_to_bytea'::bytea, and check the return value. BUT that requires a round-trip to the server....