How to quickly tell an SA.316 from an SA.319? What are the major visible differences between an SE.313 and an SA.318?
Obviously the main differences are in the installed engine; Artouste versus Astazou respectively in each case. But there is seldom any discussion of how to distinguish one from another and this is complicated by magazines and websites that mis-label photos.
The main consistent difference, which is quickly discernible, is the location of the electrical generator ( or alternator ).
In the Artouste, as shown below, the generator is forward of the intake and overhangs parallel to the driveshaft.
The generator mounting and gearing can be seen on this cutaway of an Artouste in the SAAF Museum in Swartkop ( cropped from Creative Commons BY-SA image linked here ). This example is missing the reduction gearbox casing.
The Astazou has the generator inboard of the engine and the intake and reduction gearbox form a forward-facing cone. In general it is much more "pill-shaped" than the Artouste:
The following cutaway from this image in the Flight archives shows the accessory gearbox and generator tucked neatly under the engine; what the perspective doesn't show is that it was mounted to starboard.
Do not rely on the presence or absence of the intake screen. Although most Astazou intallations do not have a port-mounted screen it is an option.
From a distance, then, look aft of the rotor mast for an object protruding above the gearbox. If it's there, you're looking at an Artouste-powered Alouette. Determine the exact model by reference to its family.
Here's a worked exercise: Alouette III aeroframe with the electrical generator clearly visible above my head. So this is an SA.316!