The question is quite generic. What are the points that should be kept in mind to identify a valid use case? How to approach a use case?
A use case identifies, with specificity, a task or goal that users will be able to accomplish using a program. It should be written in terms that users can understand.
Wikipedia's description is overly formal. I'll dig through my other texts shortly. In contrast, the original wiki's article is much more accessible.
An early article by Alastair Cockburn, cited positively by The Pragmatic Programmer, contains a good template.
This question, from just a few days ago, is very closely related, but slightly more specific.