Is this a QA question? Sounds more like a Business Analyst question. Them folk are more familiar with the requirements gathering process than us'ns here in QA/QC.
Perhaps your question is rather what should be tested for?? It would seem the focus should be on reliability and accuracy of the biometry. You would want to try different ways of faking it out. For example, use a cats-paw instead of your own thumb (only half jesting [img]images/icons/wink.gif[/img] here).