Not quite, hold it upside-down with the cap on, for convenience you can keep the hose on. The idea is to use the head thus provided to mimic the force generated by the thermosyphon effect. Use a bucket of water, most seem to be marked in litres so you have a known quantity. Pour it all in as fast as it will go and time how long it takes to stop flowing through. If a bucket proves too little to get a reliable time the flow is probably OK anyway but to be sure you could fill it with a hose for a known time and fill buckets with the outflow. Do a before and after and see if there is any difference.
Some garages/workshops have facilities for pressure flushing so it might be worth asking around. Do any neighbours, (if any are still on speaking terms with you yowling around the block that is!), have one of those pressure-washing wand things that are used for cleaning algae off concrete and paths? A good radiator should be able to stand several pounds of pressure, if not it’s on the way out anyway so better to find a weakness at home than 50Km away, there is little risk. Failing that if you have a greasy spoon nearby you could always nick a radiator off a 16 wheeler while the driver is feeding his face. It’ll be down to you how you see over it though!