You really only have to put you hand on the rads, in particular where the flow and return connections are. If you find the return is too cool you know it is not getting quite enough flow through the rad and visa versa. It does not mean you have to tweak the valves on that particular rad but gives you feedback on what needs balancing.
Those twin thermocouple devices can do the same thing as long as the probes are properly attached. There are versions with pipe clamps to secure the probes correctly. If the probes are not connected correctly it will be more nuisance than help because they will not be reading properly.
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