Take a look at your monthly statements to see how your credit cards are being billed. Look for any suspicious charges that you might not recognize. You can ask to get your card frozen if you suspect that it has been stolen or its identity was compromised.
The great idea is to check your cards in real time online. Review your cards on occasion while online. You can use a good check to see what the latest charges on your card were. This gives you enough time to get any issues you have disputed as soon as possible.