How to check critical updates in salesforce org

• Critical Updates

Salesforce periodically releases updates that improve the performance, logic, and usability of Salesforce. When these updates become available, Salesforce lists them in Setup at Critical Updates and displays a message when administrators go to Setup.

Note:

Salesforce recommends testing each update by activating it in either your Developer Sandbox or your production environment during off-peak hours.

• Steps

1. Go to Setup -> Type Critical Updates in Quick Find Box
2. Click on Critical Updates 

• From this page, you can view the summary, status, and auto-activation date for any update that Salesforce has not permanently activated.
• To view more details about the update, including a list of customizations in your organization that the update might affect, click Review.

• If an update has an Activate link, click it to test the update in your sandbox or production environment before Salesforce automatically activates it.

• References

Critical Updates