Testing /flushing radiator off vehicle

Last year I picked up a Series radiator at Newbury for a fiver. It looks like it wasn't fitted to a vehicle very long before I bought it.

There aren't any signs that it was leaking anywhere. I have read that you can test for leaks without pressure testing equipment easily. It was suggested to fill the radiator with water and lay it flat overnight. Apparently any leaks will likely show up that way.

Also, what would be the best way to flush any muck out of it before use? I have read that you can simply spray a garden hose into both the inlet and outlet. Is this right?

Thanks for any replies.