The prosternum can be quite a difficult feature to find on a tachinid – partly because many British workers, used to Belshaw’s key will never have had to find it unless they have used the European or Palearctic keys. Here is a nice photo and the prosternum is the thin, dark, vertical strip of chitin in the middle of the picture, under the flies chin and between and slightly in front of the front coxae. Either side of it is a membranous area.
The keys nearly always ask whether the prosternum is hairy or bare – in the following photo the prosternum is bare but hairy examples have 1 or more fine, black hairs along the lateral edges.
… and here is an example with a hairy prosternum – it is very difficult to see clearly but you should just be able to see small dark hairs towards the edge of the prosternum.
A combined image, zoomed in to show the hairs or lack of. Try to ignore the bristles on the coxae – under a microscope it is a little easier because you have a greater perception of 3-dimensions and the ability to move the specimen to the best orientation:
One of the big impediments to viewing the prosternum is usually poor pinning/mounting, which allows the head to drop or the front coxae to close together and block the prosternum from view. I have noticed this most often when specimens are pinned directly and dorsoventrally. If you pin a specimen laterally the head is less likely to drop down and if, in the rare event that it isn’t in a good position, you can easily manipulate it and use micro-pins to hold the specimen in position while it dries.