How do I know if my wife has access to my iCloud account

You can look in "Find My" and see which devices are logged in to iCloud and make sure that only your devices are shown.

You can go log in at iCloud.com and then log out of all browsers. After this you'll get an email every time you, or anyone else, logs in to iCloud.

To sign out of iCloud.com

Click your Apple ID photo or  in the top right-hand corner of the iCloud.com window, then do one of the following:

• Sign out from the browser you are using now: Choose Sign Out.
• Sign out from every browser where you are currently signed in: Click iCloud Settings, click Sign Out of All Browsers, then click Sign Out.