There was a recent discussion about how nearly every computer operating system could be "powned" by the NSA, and how you should use "my favorite supersecure OS" to avoid the problem.
This is somewhat unhelpful.
There is a basic principle of computer security - no access means no vulnerability. An attacker can hammer away at a machine and if it doesn't respond - then he/she/it doesn't get anywhere. (I'll make this more formal after the orange do-dad)
How then do you check if your machine responds to outsiders? There's a relatively easy to use tool for this - nmap. (short unixy way of saying network mapper).