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Qualitative analysis can be performed by extracting the alkaloids, and performing thin layer chromatography and/or by gas chromatography/mass spectrometery. Thanks to the FBI forensic lab, these methods are outlined in detail: http://www.fanaticus.com/forensic.htm
I'm not sure how one goes about the quantitative analysis though. I suggest you get a book on Analytical Chemistry, to help you better understand the methodology/terminology (something I have yet to do).
I would say send your sample to a lab to have them determine the percentage; however, I'm not sure the DEA allows labs to release quantitative data, only qualitative*...so, you may be out of luck unless you do it yourself (which will likely require access to a university lab).
*this is supposedly to prevent manufacturers of illicit substances from getting free 'quality control,' so I'm not sure if this applies to mushrooms which you can easily claim were discovered in the wild...