Check to see if the logged in member has registered for this event, then display a message on the single event page. Requires Event Espresso 4 + the WP User Integration add-on.

ee_wp_user_has_registered_before.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_action( 
  'AHEE_event_details_before_post',
  'my_add_content_event_if_logged_in_registered', 
  11
);
function my_add_content_event_if_logged_in_registered( $post ) {
  if ( is_user_logged_in() && is_singular() ) {
  
    $user = wp_get_current_user();
    if ( ! $user instanceof WP_User ) {
      return;
    }
    //is there an attached EE_Attendee?
    $att_id = get_user_option( 'EE_Attendee_ID', $user->ID );
    if ( empty( $att_id ) ) {
      return; //bail, no attached attendee_id.
    }
    //grab contact
    $contact = EEM_Attendee::instance()->get_one_by_ID( $att_id );
    //if no contact then bail
    if ( ! $contact instanceof EE_Attendee ) {
      return;
    }
    // get events for this user
    $events = $contact->get_many_related( 'Event' );
    // build an array of event IDs
    foreach ( $events as $event ){
      $user_event_ids[] = $event->get( 'EVT_ID' );
    }
    // get this event's ID
    $this_event_id = $post->ID;
    // look for a match
    if( in_array( $this_event_id, $user_event_ids ) ) {
      echo '<h3>You\'ve been here before!</h3>';
    } else {
      echo '<h3>Welcome ' . $user->user_nicename . ', please register.</h3>';
    }
  }
}

Where to add this code?