How can I find preinstalled apps via adb? Is there a way to find for example a preinstalled WhatsApp? 

So before I delete WhatsApp I can check if for example Samsung has a pre-version installed on the device (and so I can’t fully remove it)? 

With the normal „path“-command I got the actual WhatsApp path! But the Preinstalled WhatsApp-Path must be somewhere different!

Greetings 

Dossy
adb shell pm list packages com.whatsapp
adb shell cmd package list packages com.whatsapp   # works on Android Nougat and above


Any of those commands would show whether WhatsApp is installed or not.

To know where the APK is installed, run:

adb shell pm path com.whatsapp


To get all the paths where both user installed APK and inbuilt APK is, run:

adb shell dumpsys package com.whatsapp 


Look under the category 'Packages:' and 'Hidden system packages:'. The line with ResourcePath shows the path of the APK.

If your intention is merely to ascertain the existence of that app, then using adb is an overkill. An app can easily achieve the required objective. See my answer here for that.