The quickest way to know for sure is to call Sandisk customer support using the number on the packaging and quoting the serial number on the card to them. They have a database of many fake serial numbers in circulation. If your CF card doesn't have a s/n printed on it at all, it will be a fake I'm afraid.
Here's a guide including photos of a specific fake Ultra II vs a real one:
Fake SanDisk Compact Flash card
As you can see, the differences are subtle and without a real one to compare it with, you probably wouldn't notice anything was untoward.
... but there's no guarantee that the fakers haven't improved their game even further since those photos were taken, so the best way is to check with Sandisk customer support.