I would suggest that you 'map' the radiator. Take readings vertically every 4" in 4" 'strips.' Then, look at the readings for uniformity.

I would expect higher readings on the upper rows, and lower readings on the lower rows. You might also get readings a bit cooler in the center, where the fan pulls air over the radiator.

Also, you can try this as it first warms up, looking for where the radiator first begins to warm up. If a vertical coil is plugged, it will warm up slower.

Best Regards,
Jim