The iOS app, CovPass Check, can be used to verify other people's digital Corona-Impfnachweise. It scans a QR-code generated by the Robert-Koch-Institut, and displays the person's name and birthdate. This app is free and doesn't store any personal data. Instead, it simply shows them the information they're looking for. The free version of the app does not require an internet connection, and it does not store any of the data collected.
The app's name is a little bit confusing. The full name is CovPass, and it's not very intuitive, so many people aren't familiar with it. You have to make sure your version is up to date. The latest version will not work on older Android devices. Moreover, the app will not run on any older version of iOS. This means you can only use it on devices that have iOS 12.
You can download the latest version of CovPass Check from the app store for free. Most of the Windows downloads will work on both 32- and 64-bit versions. The portable version will not write to the Windows registry, and can be started from a USB stick. The main disadvantage of the app is that it cannot be used as a replacement for the paper version. It also does not have a built-in Impfschutz feature, so you may have to print it out.