how?

Common sense, plugins like nocheat logblock, vanished admins,...

Well, unless you are a server admin, there is no actual way of knowing. I need to know what perspective you want to be able to tell if someone is hacking from. If you're just a normal player; there is almost no way unless you see them doing an obvious hack. Such as; flying, jesus mode, etc. Those are server sided, and can be seen by anyone. Now, if someone is using XRAY, it's client sided so you can't really tell unless you follow them. They'll almost always go to ores.

Now, if you're a server admin.. Well, it's the same way. Unless you have a plugin such as NoCheat or NoCheat++, you can block people from using obvious hacks, and if they try it sends you a "report".