Hacked email

There isn't a definite way to tell unless you notice unusual activity that you didn't initiate. As a precaution, change your password. If this is your Apple ID, you can also add 2 Factor Authentication to your account for more security. If the attempts continue, you might want to consider a name change.

Link to contact Apple Support in the article.

Apple ID has been compromised.        

 Apple ID Password Change.     

Note the restrictions in the article.

Apple ID Name Change.        

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

Apple ID -Two-factor authentication          

 Two Factor Authentification availability.     

 Apple ID - Using app-specific passwords.     

Apple ID - Two Step Verifications FAQ    

Some of my links also are given by Eric above.

How can you tell if they are trying to hack into your iPhone? Have your seen a message that someone is trying to sign into your account from a different location? That's not iron-clad evidence of hacking, but:

If yes, then changing your password is a reasonable thing to do. See What to do after you change your Apple ID or password https://support.apple.com/en-us/HT205075) (both links given by Eric)

Check your device list to see if there is a device signed in that you don't recognize using Check your Apple ID device list to see where you're signed inhttps://support.apple.com/en-us/HT205075) Remove any device that shouldn't be signed in using the steps in the link. Someone may be trying to hack your iPhone as you say.

If there are none that shouldn’t be signed in, then you should not have a problem at this point. But add two-factor authentication (https://support.apple.com/en-us/HT205075) if you aren’t already using it, and it didn't hurt to change your password. (see what Eric said.)