Only real way to tell is google your username and see if there's any activity on other servers you've never been to. That isn't foolproof though. The only really foolproof way is to maintain control of your e-mail, have a strong password set for your account, and make sure to keep a copy of your receipt for the account in an easily accessible place in case you do lose control of it at some point and require assistance from mojang to regain control. As far as the specific context on EMC, I'll speak more broadly since players could come across this that had this happen to them. If you become aware of a situation where your account could be compromised, notify staff immediately. We have methods of preventing access to accounts that are not authorized and can protect your account from disallowed activity until you regain control of it.
It depends a bit on what kind of account we're talking about, yours or someone elses. Generally speaking: If you have a Minecraft account yet you cannot log onto https://account.mojang.com/ then something fishy is going on. Also: if you got your Minecraft account through other places then Mojang then once again something fishy could be going on. Next what Elfin said, but to expand on that... Definitely keep your account safe by using a good password. I personally recommend using a program like PWGen for this (generating the password). Then, very important, don't use this password anywhere else but your Minecraft game. Not even on the Empire (even though I actually trust EMC's system to keep it safe (and with safe I also mean keeping it safe & hidden from the devs. themselves)). Also: be sure to set up Mojang's security questions, and don't fill out questions which people could know about you. Instead make stuff up. You don't have a pet but you always wanted one? That could be nice security question to use, especially if you never shared a possible name with anyone. Stuff like that. (edit): Also: consider changing your password ever once in a while. Every 3 or 4 months or so, sometimes together with your security questions. Even if things were compromised then it would immediately get a whole lot more difficult for the attackers again. Hope this can give you some ideas.
