The easiest way to tell if your SIM card is no longer active is if you completely lose service on your phone. You may receive a text message stating the SIM card for your number has been changed, and to call customer service if you didn't make the change. But with your SIM card no longer active, you won't be able to place a call from your phone -- not even to customer service (more on this below).
In short, the quickest way to tell if you've been affected is if your phone completely loses service and you can't send or receive text messages or phone calls.
The truth is, if someone wants access to your phone number bad enough, they will do all they can to trick your carrier's support representative. What we've outlined above are best practices, but they're not foolproof.
Researchers were able to pose as account holders who had forgotten their PIN or passcodes, oftentimes providing the recent numbers called by the account holder. How do they know those numbers? They either tricked the account holder into calling a couple of numbers -- or even scarier, phone numbers for incoming calls to the account they want to take over, meaning the bad guy simply needed to call the target's phone number themselves.
Once you realize you've lost service on your mobile device, call your carrier immediately and let them know you didn't make the changes. The carrier will help you recover access to your phone number. I can't emphasize this enough -- do not wait to call. The longer someone has access to your phone number, the more damage they can do.
Here are the customer service numbers for each major carrier. Put your carrier's number in your phone as a contact:
With your SIM card deactivated, you won't be able to call from your phone, but at least you'll have the number handy to use on someone else's device.
You'll also want to reach out to your bank(s), credit card company, and double-check all of your online accounts to make sure that the perpetrator hasn't changed your passwords or made any fraudulent transactions. If you find transactions that aren't yours, call your bank or visit a branch right away and explain the situation.
Remember, no matter how many PIN codes or passwords we add to our online accounts, there's still a chance that someone will find a way to break in. But at least by setting a passcode for your account, and knowing what to do if you find yourself a victim of SIM swapping, you're prepared.
Another critical aspect of strong online security is to use a password manager to create and store unique passwords on your behalf. Additionally, enable two-factor authentication on every account that offers it.