Is there some place you can take or send in supplements for testing if you have concerns about purity or the contents? Such as the FDA or a consumer fraud agency, or perhaps even a local pharmacist or physician?
You’d have to find a lab willing to perform the test at a price that you are willing to pay. A pharmacist may be able to recommend a lab, but I doubt he would have the necessary equipment on site. A medical that does blood work might also be able to test the vitamins.
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Testing however is only of limited value unless you intend to test every batch. Companies won’t necessarily source their ingredients consistently and even if they do, those sources may not be the primary and may make changes in the manufacturing process.
A notable example is what happened with L-tryptophan in the the late 80’s IIRC. Showa-denko made a slight change to the fermentation process of the bacteria used to make the amino acid which resulted in a cross-linked form that caused problems for many people resulting in a rare illness called EMS (eosinophilia myalgia syndrome) and a few people actually died as a result. The x-linked idea was the theory at the time when I spoke to people at the FDA, but wikipedia doesn’t seem to mention it.
edit: the best thing to do is use a supplier like Solgar the represents to using GMP (FDA good manufacturing practices) and either sources their own ingredients or tests them via GCMS (gas chromatography-mass spectroscopy)