This is kind of bug, but you you can't read config 999 from script (edit: incorrect, see Phil's comment below), and type(call_event_proc) ~= "function" ) hasn't worked since Lua was turned into a module.
If you really want to know, you can try to call a no-existent or harmless eventproc inside pcall, like
if not pcall(call_event_proc,'check_chdk_native_calls')) then
print('native calls not available!')
end