Can projects make certain guesses, such as all school pupils are from postcodes in catchment area? Or should we survey each pupil?
Generally, you should use the information you need to gather anyway and incorporate monitoring into that. If you ask people for their address, you will probably be able to tell if they are from a disadvantaged area without having to ask them. If you work with school children, collaborate with the school office to find out if all of your participants are from the local disadvantaged area. If you have people register for activities, using the registration form on this website (see Monitoring and then Tools), then you will capture the information you need.