Create two pointers, and set both to the start of the list. Update each as follows:
while (pointer1) {

pointer1 = pointer1->next;

pointer2 = pointer2->next;

if (pointer2) pointer2=pointer2->next;

if (pointer1 == pointer2) {

print ("circular");

}

}

If a list is circular, at some point pointer2 will wrap around and be either at the item just

before pointer1, or the item before that. Either way, its either 1 or 2 jumps until they meet.