Hello Clare Perry,
Welcome to the Microsoft Community.
I understand that you want to know how to identify emails that have been replied to.
In New Outlook, newly replied emails indeed do not immediately show a prompt, but you will see the prompt for previously replied emails.

Actually, you can identify them by the icon next to the email. Emails with a left-pointing arrow have been replied to, while those without it have not. You can observe a similar design in Classic Outlook.

By the way, Classic Outlook and New Outlook are two separate products. They are not old and new versions of the same application. Classic Outlook is a subscription-based paid application, while New Outlook is a free application. Therefore, Classic Outlook has more features and is more stable compared to New Outlook. If you have a product subscription, it is recommended to use Classic Outlook rather than New Outlook."
Please feel free to let me know if you need any further assistance.
Best Regards,
Zachary.W - MSFT | Microsoft Community Support Specialist