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In the spirit of "teach a man to fish", here's how I work it out. This works for any engine, any thermostat, and even if you aren't sure of the direction of coolant flow. Hold the thermostat in your hand and think about how it works. The bimetalic element (that opens and closes the valve allowing coolant flow) must be immersed in the engine coolant whose temperature it is to regulate. While the engine is still warming up, and the thermostat is closed, there may not be any coolant on the downstream side of the thermostat and/or the temperature on that side will be near ambient, and the temperature regulation will not work as it should. Now find the bimetallic element. It is typically of substantial size and is usually contained in some sort of cylindrical "slug". It couldn't be the spindly spring or rod or support assembly usually comprising "the other side" of the thermostat. For the N-series thermostat. old or new, this is quite obvious, even though the current replacement thermostat looks a lot different than the 60 year old original production part. For a "new production replacement thermostat" there is one side which has a round can containing the bimetallic element, and this side goes towards the engine. The other side has a spring and a triangular support assembly. If you view this triangular support assembly as being arrow shaped and that it should be assembled so the arrow "points to the radiator", then the rule of thumb you offered would work. The original production thermostat does not have the same triangular support assembly, and I'd be hard pressed to figure out which end "points to the radiator". But it is easy to find the bi-metalic element, and put that end close to the engine.
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