Let's start by assuming the computer is on a LAN, behind a router running NAT, as is the case in most homes and small offices. If that's the case, then any computer on the WAN side of the router would have a difficult time connecting to the computer, being that NAT typically does not forward incoming connections to hosts on a LAN by default. Notwithstanding, you might want to run an NMAP on the computer, from another computer on the LAN, just to be sure that the computer in question doesn't have any ports open for accepting incoming connections.
That leaves the possibility that there is a program on the computer that is making outgoing connections, to some host server on the WAN side of the router, which is then used to remotely access the computer. If your router allows you to view outgoing connections, you might want to take a look at these, and see if there are any outgoing connections from the computer to hosts/ports that you don't recognize.