When SIM switching happens, you can see the event in System -> Logs. You will see the time when the SIM switch happened, to which SIM card did it switch, and the reason why it switched.
You can configure SMS and e-mail notifications in Services -> Events Reporting. The service is quite flexible. However, currently, there is no way to trigger an event when the SIM switching happens. You can use a workaround like setting the event type to mobile data and sending a notification when the status changes to 'connected'. Include 'SIM slot in use' parameter in SMS message. This way, when SIM switch occurs and the device reconnects with a new SIM, you will receive an SMS message containing the information regarding which SIM card is currently used. You can find more information HERE.
Not all SMS notifications are in the events reporting. For example, if you go to your mobile interface settins in Network -> Interfaces -> your mobile interface, you can see a mobile data limit section in the bottom. If you enable that option, you will be able to set data limit and configure a SMS notification when a specified amount of data was used. Information HERE.