The old "tortoise and the hare" problem. Essentially, the "fast" pointer moves 2x the speed as the "slow" pointer. If there is a cycle, they will meet.

int HasCycle(Node head) {
    if (head == null){
        return 0;
    }

    Node slow = head;
    Node fast = head;

    while (fast != null && fast.next != null){
        slow = slow.next;
        fast = fast.next.next;

        if (slow == fast){
            return 1;
        }
    }

    return 0;
}