Find the installed date
Open Settings and the device or software information screen, then locate Android security update or security patch level. Menu names vary.
Understand the date
The patch level names the bulletin level included in the build. Installing today does not necessarily make the displayed date today.
Compare carefully
A bulletin describes fixes available to manufacturers; it does not prove a particular model or carrier build received them.
Primary sources
Important
Device support and safety information can change. Recheck the linked manufacturer and regulator evidence before making a purchase or safety decision.