Using a set of demographics (e.g., gender, age, place of residence, income class, etc.) will be the most basic approach in determining the target segment. Keep in mind that some general statistics for these demographics can be found online. If you want to go into more details, then you might have to look into other specific classifications that are relevant to the market you are targeting such as the psychological profiling and lifestyle preferences of the different customer segments. 

In this regard, the entrepreneur must be able to do actual field research like surveys, focus group discussions, in-depth interviews, observation techniques, etc.