this is repost from level 2 
You will not be able to check 100% whether accounts exist without trying to set up one.
EDIT:
https://pastebin.com/wSAGu2Tb
This is a simple php script that checks the names given in the file.
Save the script as check.php , create the accounts file in the same directory and enter in it the usernames to check each username in the new line.
Script written quickly without checking, I do not know if it works on the windows and how it works for a large number of accounts (I use something more sophisticated with the name generator 
). If the ig starts blocking, you can try to uncomment the sleep function and change the useragent in the code.
Good luck
PS. source: https://mpsocial.com/t/wtb-instagram-username-availability-checker/48409/4