Password with libpq

I'm setting my PostgreSQL server to demand md5 encrypted passwords
from users. In my C programs (using libpq as the interface), it's no
big deal to pass the password through PQconnectdb.

However, what about in the general case where my client program
doesn't know if a password is required or not. Is there any libpq
function to query the server if a password is required? Perhaps,
there's a return value for PQconnectdb which would tell me that the
connection failed because of the password? (In which case, I can ask
for the password and try the connection again.)

Thanks.
-Tony