how can I tell if my computer files have been accessed from a hacker

Hello John B,

Welcome to Microsoft Community.

I am a Microsoft user just like you, I hope you are doing well?

Did you mean hackers accessing your local files remotely? Did you grant anyone remote access?

If you mean your OneDrive files, did you receive any notification for unusual sign-in activity?

You can check your Microsoft account sign in activity. Using Sign-in activity, you can see when and where your account has been accessed—including successful sign-ins and security challenges—on the Recent activity page.

Check your sign-in activity: https://account.live.com/activity

Below are few steps recommended by Microsoft to keep your account safe:

Create a strong password for your account. Use the Microsoft Authenticator phone app to sign in without a password. Make your account easier to recover. Make sure your operating system has the latest updates. Never reply to email asking for your password. Check your recent activity. Keep your devices safe.

See details: https://support.microsoft.com/help/628538c2-7006-33bb-5ef4-c917657362b9

Learn more: https://support.microsoft.com/help/eba43e04-d348-b914-1e95-fb5052d3d8f0

Please let me know if you need more assistance.

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Warm Regards

I would probably start by scanning my device for viruses with Defender or any other anti-virus you have.

It is not likely that they accessed your computer without a malicious application being installed on your PC.