How do I know which version of the POS app my tablets are running?

Open Settings → The POS and look at the box called The app. It shows the current version beside the Download the app link. When a newer version is published, each device shows a banner saying a newer version is ready, with Update now and Later. The update takes about twenty seconds, the device cannot sell while it installs, and it stays paired afterwards. The POS and the counter tab in the docs covers updating the app.