The easiest (safe) way for occasional use is to create a dial-in account using software like pcAnywhere (Norton) or Windows remote desktop. Connect the SCADA to the phone line, and dial in from a remote location from a computer with the remote client software installed, then you have access to the PC desktop and can see everything a local user can. I use this technique for quick troubleshooting for our international sites.
Note that this technique suffers from long screen update times, but it's very secure and requires little effort to set up. If you need more performance you will have to set up a broadband internet connection, but this requires a lot more setup and hardware to keep your production machines safe from internet nasties.