How To Check Whether User In Lightning Experience Or Classic Experience In LWC

Please use the below code in your LWC components under the java script controller.

ExperienceCheck.js

import {api,wire, track} from 'lwc';
export default class ExperienceCheck
{
 checkTheExperience(event)
 {
  if(document.referrer.indexOf(".lightning.force.com") > 0)
  {
   console.log('user in lightning experience');
  }
  
  else
  {
   console.log('user in classic experience');
  }
 }
}

Hope this helps you..Enjoy..!