It's tough to say (definitively) if it's factory sealed due to the prevalence of shrinkwrap machines, as folks have pointed out (and note that some thriftstores/used stuff dealers actually own such machines and will shrink-wrap many items before putting them out on the shelf just as a convenience (keeps everything together) - if many other items in this store are shrink'd, that may be what has happened). The manual or other paperwork would be the most obvious way to determine if it's original - if it has all of the original paperwork, and none of it is rumpled, damaged, discolored, etc that's a good sign. You can also check if the disc looks brand new - any scratches, fingerprints, etc are a bad sign.
But in general I'd say get it and open it and enjoy it - like Auzner poined out, Sim games are fairly common, so you aren't likely to be cashing in on big bucks here even if it is brand new sealed.