NBTTagUtils#getAttribute(ItemStack item, String attributeName);

NBTTagUtils.java

    public static Object getAttribute(ItemStack itemStack, String attributeName)
    {
        // CraftItemStack.asNMSCopy(ItemStack item).

        Class<?> obcCraftItemStackClazz = ReflectionUtils.getOBCClass("CraftItemStack");

        try {
            Object nmsItem = ReflectionUtils.callMethod(obcCraftItemStackClazz.getMethod("asNMSCopy", ItemStack.class), null, itemStack);
            Object compoundGetter = ReflectionUtils.callMethod(nmsItem.getClass().getMethod("getCompound"), obcCraftItemStackClazz, null);
            return ReflectionUtils.callMethod(compoundGetter.getClass().getMethod("get", String.class), compoundGetter, attributeName);
        } catch (Exception nbtTagUtilsAttributeGetterExceptions) {
            nbtTagUtilsAttributeGetterExceptions.printStackTrace();
        }

        return null;
    }