Is there a way to check the default web browser that is set up on my smartphone? I want to use "Chrome Custom Tab." 

However, to use "Chrome Custom Tab," the default Web browser must be set to "Chrome".

Is there a way to determine which default web browser is being used on your device?
you can check standard browser from app settings. this is for android 7 (german translation)

Settings -> Apps -> Chrome (App info, scroll down) -> Browser App -> Browser App (again)