Using IDA to find pointer passed to procedure

Question

Loading the source into IDA, I have this function:

According to the source for Lua, this function should be passed a pointer to the Lua_State struct:

LUA_API int lua_gettop (lua_State *L) {
  return cast_int(L->top - L->base);
}

I'm currently trying to figure out what pointer is being passed to this procedure (In this case arg_0). I can attach a breakpoint and it correctly breaks, but how would I find the associated .data location for where it's stored?