How to determine if ATMEGA328P is using internal or external clocks?

Can someone please shed some light on the subject? I very simply want to know how to check if my ATMEGA328P is using its internal or the external I have set up on pins 7 & 8 respectively.

Thanks in advance!

Read the fuses. The exact procedure will depend on the type of programmer you are using.

Read the low fuse with an isp programmer.

Or ground pin 7 while it's running and see if it stops :P

Run this sketch with and without the OSCCAL statement commented out. If you’re using the internal oscillator it will affect the blink rate.

#define LED_PIN 2        // LED and resistor between digital pin 2 and ground

void setup() {
  pinMode(LED_PIN, OUTPUT);
  OSCCAL = 0;


// 1 Hz blink
void loop() {
  digitalWrite(LED_PIN, HIGH);
  digitalWrite(LED_PIN, LOW);

Or ground pin 7 while it’s running and see if it stops.

Well, that was simple, wasn’t it?