I can spot Simosyrphus grandicornis quite easily but sometimes to explain to someone else can be a little difficult. I have had a look at the two species side by side from the collection and I think there are a few easy characters which you should be able to see in most photos. The character that first separates the genera in the key (Vockeroth's 1969) is Simosyrphus has a distinct margin on at least tergite 5 and in Melangyna (Austrosyrphus) this is absent. This character probably isn't visible in most photos and it is always hard to distinguish a character when you don't have an example of its presence to compare it to. There are a number of species of Melangyna (Austrosyrphus) in Australia and they are a bit hard to distinguish easily (not to mention there are probably a few species undescribed just to muddy the waters even more).

So a few characters to spot the difference between the two species you have asked about in a photo. I checked the website you posted your photos to and found the original enquiry and based on Susan's photo (which is definitely Simosyrphus grandicornis) you can see a yellow scutellum, the yellow face/frons, and the yellow lateral stripe on the thorax all of which M. viridiceps doesn't have. The shape of the abdomen is different - S.grandicornis is fairly bulbous and M. viridiceps is parallel sided and the legs are mostly yellow in S. granidicornis and all black in M. viridiceps.

Some species of Melangyna (Austrosyrphus) have a darker yellow face and scutellum but none of the ones I have checked in the collection have that yellow lateral stripe. The abdomen shape can vary a little.