Simultaneously go through the list by ones (slow iterator)
and by twos (fast iterator). If there is a loop the fast
iterator will go around that loop twice as fast as the slow
iterator. The fast iterator will lap the slow iterator
within a single pass through the cycle. Detecting a loop is
then just detecting that the slow iterator has been lapped
by the fast iterator.

This solution is "Floyd's Cycle-Finding Algorithm" as
published in "Non-deterministic Algorithms" by Robert W.
Floyd in 1967. It is also called "The Tortoise and the Hare
Algorithm"

The above method is working of course, but is not the most
efficient. Other methods are however quite complex and not
so easy to explain.
Anyway, to exemplify this aforementioned method, maybe not
the most efficient code, but off the top of my head, hope
there are no misspellings.

bool isCyclic(LinkedNode *list)
{
if(list == NULL || list->next == NULL) return false;
LinkedNode *node1 = list, *node2 = node1->next;
while(node1 != node2)
{
  if(node1==NULL || node2==NULL || node2->next == NULL)
return false;
  node1 = node1->next;
  node2= node2->next->next;
}
return true;
}

NOTE: the assumption is that for noncyclic list, the last
node has next pointer set to NULL.