Formulating a questionnaire to estimate students' family income

Hello,
I am preparing a questionnaire for an economic experiment with university students.
I need to know their family income, without having consistent access to the students demographic details.

Assuming that some students will not be aware of their specific family income, can anyone suggest me what I should ask them?
I thought about number of rooms of their home per total number of family members, but I think there could be better questions to ask them to get an estimate of their family income.

Any suggestion?
Many thanks!

Ask for parent's occupation. For most CS and PS jobs and for some other jobs there is public information that would allow you hazard a guess from the occupation

Hi Snap
If you ask for both parents occupations you can then get information from the CSO website in relation to occupations and social class, that is used for the census as well as other reports and use social class rather than income.