cancel while waiting for a lock - how?

Hi,
I notice that in version 7.0.3 there has been a fix to allow cancel
while waiting for a lock.  What I would like to know is how to cancel
an sql statement that is waiting for a lock or how to even know
that I am waiting for lock. Better yet, how can I specify that I
would prefer the the statement to return immediately (without aborting
the whole transaction) if it cannot aquire the required locks. I would
like to be able to do this using libpq.

Thanks
Kevin English