iOS: Apple added a few new things in the iOS 10.3 update, and one of those is a tool to identify 32-bit apps that may be rendered obsolete in the next version of iOS.
Image via Wikipedia.
Head to Settings > General > About and scroll down to Applications. If you can tap this button, you're taken to the app compatibility page, which is essentially a graveyard of ancient apps on your phone. If you see any apps here, it means they haven't been updated with 64-bit mode, which was introduced way back in 2013. If you can't tap the Application section, that means all your apps are up to date.
SensorTower compiled a list of the nearly 200,000 incompatible apps in the App Store and the bulk seem to be games or education apps. If you see an app here you really care about, or worse, that you need for work, reach out to the app's developer and see if they can update it with 64-bit support.