To determine if there are any results you can do any of the following:
if ($mentor->first()) { }
if (!$mentor->isEmpty()) { }
if ($mentor->count()) { }
if (count($mentor)) { }
if ($mentor->isNotEmpty()) { }
Notes / References
->first()
https://laravel.com/api/5.7/Illuminate/Database/Eloquent/Collection.html#method_first
isEmpty() https://laravel.com/api/5.7/Illuminate/Database/Eloquent/Collection.html#method_isEmpty
->count()
https://laravel.com/api/5.7/Illuminate/Database/Eloquent/Collection.html#method_count
count($mentors) works because the Collection implements Countable and an internal count() method:
https://laravel.com/api/5.7/Illuminate/Database/Eloquent/Collection.html#method_count
isNotEmpty()
https://laravel.com/docs/5.7/collections#method-isnotempty
So what you can do is :
if (!$mentors->intern->employee->isEmpty()) { }