millis(), how to detect overflow (after +/-50 days)?

[system]

void setup ()
{
  Serial.begin (115200);
  unsigned int end  = 0x10; // after rollover
  unsigned int start = 0xfff0; // before rollover
  unsigned int diff   = end - start;

  Serial.println (diff); // prints 32.
}

void loop ()
{
}