how do I know if my icloud acct has been hacked?

You probably wouldn't unless you noticed your data was changed. As suggested, if you think it might have been hacked, change your password.

Apple ID Password Change

Apple ID email address or password - What to do after you change I suggest doing this before you change your Apple ID or password.

If you've gotten an email stating such, you'd need to forward as an attachment to reportphishing@apple.com

Check icloud.com from a browser. You can view your info saved in iCloud there.

Finally, if you feel someone may have gotten your password, change it right away.