function rgb(i)
  local r = bit32.band(bit32.rshift(i, 16), 255)
    local g = bit32.band(bit32.rshift(i, 8), 255)
    local b = bit32.band(i, 255)
  return r, g, b
end

function bright(r, g, b)
  return (r+g+b)/(255*3)
end

function pixeldiff (u, v)
  local r1, g1, b1 = rgb(u)
  local r2, g2, b2 = rgb(v)
  return bright(math.abs(r1-r2), math.abs(g1-g2), math.abs(b1-b2))
end

w, h = img.width, img.height
hw, hh = math.floor(w/2), math.floor(h/2)
sum = 0
for y=0, hh-1 do
  for x=0, hw-1 do
    u = img.getInt(x,y)
    v = img. getInt(w-1-x, h-1-y)
    sum = sum+pixeldiff(u, v)
  end
end

sym = 1-sum/(hw*hh)

return "180° symmetry: "..math.floor(sym*100+0.5).."%"

Coded on a smartphone! :)