hosting several accounts from one account?

If you've bought hosting which allows you to host more than one site on it, can others tell if they're linked (other than through the IP)? Eg. I've bought hosting for site mysite.com. I've then included 3 add-on domains. When you install an add on domain in wordpress it asks if you want to install either as myaddondomain.mysite.com or myaddondomain.com. I've installed as myaddondomain.com.

Is there any way, other than through the IP to tell if they're connected? The reason I ask is because one day myaddondomain.com was down and the error message on the screen had lots of words but did include "myaddondomain.mysite.com" which would indicate that they're linked. Is there some way of pinging the site or something like that which shows the root directory or something like that to give away that they're on the same account?

Good question. I do the same thing for a few sites an interested in the answer. Never even thought about it before.

If you buy an IP, it allows only one domain. Multiple ad-ons are not allowed. If you try means, it will down your main site domain.

It really depends on how your host handle it. The IP on shared hosting will be the same, so you can link them. But you can also end up on the same IP as tons of other sites, so it's also harder if someone wants to specificaly targets it.

It can be easily verified by checking the hostname, the traceroute, the registrar etc of both sites.