You should always be able to get water out of your instrument. Depending on the valve set up and alignment there are seceral methods that will allow you to do it quickly and completely.
But if you have done your regular procedure and still find there is water in the instrument you need to determine where it is. How can you do this? First blow through your instrument with no valves engaged. Do you hear signs of water? If not continue this test by adding each valve individually. If you do hear signs of water before engaging the valves then you need to locate each low spot the water could be at. Look at your instrument and turn your instrument in such a fashion the water will move towards a valve, tuning slide, or bell of the instrument. By turning or tipping your instrument the water/spit/condensation will eventually make its way to an opening where you can drain it.
If this doesn't work then find a french horn player with a plumbing certificate and they can help you.