I guess you'd have to look at the time period. If you're talking about someone from the time of Moses, obviously there were no muslims at the time as we know muslims today. The Rasul hadn't come yet , and the Quran wasn't revealed. But if the person is a follower of Moses and believes in Moses's teachings as they were in their TRUE form, then of course you can consider them as a Believer. Even then, God has ultimate authority as to that person's destination.
Then if you fast forward some in time and you come across a religious individual who is blindly following the Old Testament, which by that time is changed, then I dont know really whether to put him in the True Believer box or not.
Fast forward in time some more and we reach the Rasul . His followers until today are termed Muslims. But the faith that he taught was the same that Jesus and Moses and all the other Prophets taught. Thus their followers would also be Muslims - or Believers. And since we believe the Quran has been preserved thru time w/o changes, today's believer is no less than the believer of the Rasul's time.
Anyone else would be a non-muslim.
Although I'm sure many would place people like me in the non-muslim box, and people like OBL and the mullahs who have been behind the havoc in Karachi in the believers box. Its all relative on your personal beliefs.