Right, so the most consistent and distinct features of jewish heads I've found so far are:
The forehead forms a 135° angle with the top of the head, both of which are mostly flat. For the few heads with a slight curvature to either incident, there will still be a distinct point which separates the two, measure from said point to the brow and the back of the head.
(jews tend to have a small to nonexistent brow ridge, much like the lower hominids, allowing the forehead to extend all the way to the nose ridge)
The nose bridge and the forehead slope form a 172° angle when extended to their intersection point. This is measured based on the "hook" part of their noses. Very few jews have a concave angle here, whereas the vast majority of Whites will be at least slightly concave.
When measuring from the underside of the nose (where it joins to the face; the nasal cavity) to the lower lip, and from the underside of the lower lip (the starting point of the chin) to the underside of the chin, the two lines will be parallel.
Lastly, the back of their heads almost always join into the neck at a much sharper angle, 125° to 115°, as opposed to 150° to 170° for Europeans.
Different features will vary in size or positioning, but these angles stay relatively consistent. If you assume a normal 5° of variation between individuals, this should cover all jews, and no Whites.
The most "fragile" angle is the nose bridge to forehead incident, and even there only about 20% of Whites form a parallel angle, virtually none are convex. Not a single White person will match all three, and I'd be very, very impressed to find one matching two (and also very suspicious of that person's ancestry).
All these angles are easily traced and measured over a low-res profile picture, which will often be available online, or can be obtained inconspicuously with a camera. For a low-tech solution in the coming war, two pieces of yarn and a compass will make a reliable field set for filtering civilians.