I removed my bathroom radiator ages ago, leaving pipes and identical valves in place, but without noting which was flow and return. Having re-fitted the bathroom, I now want to install a replacement radiator. With most rads in my house I could feel the temperature difference on flow and return pipes but the bathroom pipes are at the end of a run and I can't tell the difference. Is there a dodge for this that doesn't involve holes in floors or ceilings?