Devices accessing iCloud

I believe you want to check and see which devices are signed on to your Apple ID.

Open Settings ➔ Apple ID (Your Name at top) : Here, scroll down and you will see all your devices that are signed in with your Apple ID. If you see a strange device then tap on it and REMOVE the device. If you feel like someone else has access to your iCloud account then I recommend that you change the password to your Apple ID.

Axel F.

Nothing explicitly has access to your iPhone, though if you are concerned you should change your AppleID Password:

If you think your Apple ID has been compromised - Apple Support