Is the mineral octahedral or rounded grains (or massive). Hercynite is usually rounded grains or massive, not euhedral crystals (not to say it never is octahedral). It is usually green or greenish black, unlike black magnetite. But the best test is magnetic. Hercynite is not attracted to a magnet while magnetite is strongly attracted.
I am assuming you have small grains or crystals in a micro sized vug. If you insert a small cylindrical magnet in the end of plastic drinking straw, find its balance point and pass a needle through the midline of the straw at that point, you can balance it on a support made from cardstock (or any nonmagnetic material). It helps to cut a one centimetre length of drinking straw, slit it along its length and slide it onto the longer straw. You can move it as a counter poise to finely balance the assembly. Even a single micro crystal when brought up beneath the magnet will cause the straw to move.
You can visually differentiate them, Magnetite is metallic and shiny, shiny glaze is hercynite. 2) On the rocks or paragenesis, hercynite is most often associated with crystalline schists with corundum or cordierite. The best method is an EDS analysis.