While it would be nice to know conclusively whether the bag you have is going to keep you warm during the cool desert nights or in the crisp alpine air, there are too many variables to accurately determine the temperature rating of older bags. When was the bag manufactured? Was the bag stored in a stuff sack or hung upside down? Was it stored in a cool, dry place like a closet on the main floor, or in a hot garage or storage shed? If it is filled with down, what is the fill power and weight? If it is filled with synthetic insulation, what generation of synthetic? Hollofil has a dozen different generations of that particular type of insulation. What is the fill weight of the synthetic insulation?
If you are not the original owner and cannot answer all of these questions, it becomes exceedingly difficult to give you an accurate temperature rating. Also, as a general principle, bags lose their warmth as time goes on, slowly becoming less warm than they initially were tested to be. It is best to test it first close by to home, rather than risking a very uncomfortable night far away at a camping site.