I have some older e-mails that I have not used in some time. I am talking several years at this point. I do not know if they are safe to use or not. I am not sure if anyone had accessed the accounts either. I am still able to login but how can I tell if it has been hacked? I am really not sure what signs to look out for with something like this.
You can make use of this site here: https://haveibeenpwned.com/
If the password is still the same, take a look at your sent folder. If you are not noticing anything there and you are not receiving random/strange new e-mails, you should be fine. I would make use of the link above to check for sure though!
I agree about using tools like the one mentioned above to scan your e-mail just be sure that they are legit or they will sell off info tied with your e-mail address. As fare as I know, the one mentioned is legit.
Just update or change your password every 3 or so months and be sure to login to your e-mail every week and you should be fine. Also make sure of 2FA.
You will usually see the e-mail receiving things you never signed up for as well as sending out e-mails. If you ever suspect your e-mail has been compromised, changing the password should be done ASAP and 2FA should be used moving forward.