If taking cash (that's not directly from the bank), be sure to use one of those counterfeiting detection pens on it. Granted, they won't work if the fake money was make with real money. You can get these pens at Office Depot and similar places. I've never had to use them on Canadian money, though. We used them all the time at a store I worked at for any larger bills.
The OP said he couldn't tell a real from a fake $100 bill (or words to that effect). Go to a bank and I'm sure they'd be willing to show you all the things to look for. Especially if you're withdrawing that $100 bill!
Good luck!