Good question. I've struggled with that numerous times when asked to verify structural capacity for an existing floor, roof, etc.
The first thing to do is look for any grade stamps on the lumber. These usually indicate the species, moisture content and grade. Check out the AITC (American Institute of Timber Construction) for examples of stamps.
If no luck there, the other alternative is to take a sample of the wood and have it tested in a lab. I'm not certain there are any ASTM standards out there that cover this. But you can get a beam, perhaps 3 or 4 feet long and test it in flexure to get a load/deflection curve and a failure moment/shear for it. This will give you some indication of the modulus (E) and ultimate stress of the material.
Using a safety factor of 2.0, you can back into an allowable stress and compare the Fb and E to the NDS tables to find a close match.
I would also recommend basing your work on at least three samples as wood varies in properties.
Short of all that: Use a very conservative stress level such as a Hem Fir Stud grade for your calcs. Just be aware that the less you know for sure...the higher your safety factor should be.
Try contacting one of the lumber grading agencies, such as Southern Pine Inspection Bureau or Western Wood Producers Assn. They can give you a graders manual which you can correlate the wood characteristics such as holes, knots, slope of grain, and wane, to the grade.
