There are two options. Neither option really exposes "is the other app installed?" to you the developer, instead the OS handles the routing.
If the other app implements Universal Links (https://developer.apple.com/ios/universal-links/) or App Links (https://developer.android.com/training/app-links) you would simply use tecton.actions.openURL('http://www.universallink.com', );
https://cdn1.onlineaccess1.com/cdn/base/tecton/v1.4.2/Documentation/index.html#/capabilities/openURL
This will take the user out of the Q2 mobile app and into the native browser which from there (if configured properly) the browser will redirect the end user to the app (if installed) or the app store (if not installed).
This is the prefered solution for iOS and Android.
If the other app does not implement this and instead uses URL schemes. You will have to use tecton.actions.openApp():
https://cdn1.onlineaccess1.com/cdn/base/tecton/v1.4.2/Documentation/index.html#/capabilities/openApp
Note that this function is only available as of UUX version 4.4.0.90.