Otherwise known as the Humeral bristles or Humeral callus bristles, this test in the keys is sometimes difficult to grasp because >95% of the specimens you will see are negatives ... they won't have a "roughly equilateral forward-pointing triangle".
Simply put, the Postpronotal lobe or humeral callus is the rounded bulge on the antero-dorsal 'corners' of the thorax, just behind the head ... anthropomorphically speaking they would be the 'shoulders'. There are anything from 2-6 strong bristles and they should stand out from the underlying, shorter hairs. Look for the 3 strongest/longest and most posterior of the bristles and judge whether they are arranged in a roughly equilateral and forward-pointing triangle.
Remember that, most species do NOT have bristles in the shape of a forward pointing triangle and those that do are not commonly found - Nemorilla (getting rarer to find these days), Myxexoristops (very rare), Phebellia (rare) and Allophorocera (uncommon). So don’t worry too much if yours is a bit borderline – if in doubt try the negative option first. Always make sure that you are looking at the correct bristles because there can be anything up to 6 on the humeral callus but we are only interested in the 3 largest and most posterior. Here are some positive examples (with the equilateral triangular arrangement of bristles):
Allophorocera ferruginea (lateral)
Nemorilla floralis (dorsal)
and here are some without a triangle:
Bithia demotica (lateral)
Linnaemya picta (anterolateral)
Did you see the difference? The problem comes in that it is possible to make a triangle out of any 3 bristles but remember that in species without the triangle the middle (anterior) one is usually in line with the other 2 or only slightly pushed forwards or it can be offset to one side. But in species with the triangle the middle one will be pushed strongly forward and usually midway between the 2 posterior ones - sometimes this is described as having the middle bristle pushed forward by more than twice the width of its basal socket, but I'm not sure whether this adds much to the debate really.
Once you get your eyes in you should be able to spot the triangle with the fly in any orientation but to start with try holding the fly so that you are looking down on the dorsal surface with the head pointing away from you. Tilt the fly slightly right and left to examine both sides before you make your decision.