車牌號碼 - car license no.

u can click into the link under below :

http://www.gov.hk/tc/residents/transport/vehicle/regmarks.htm

then go for the path :

Online Enquiry about availability of vehicle registration mark and application for reservation of traditional vehicle registration mark for auction

after that insert the no which u want to know being occupied or not no matter is a traditional no neither nor a sepcial personalized vehicle registration no.

Go to the Licensing Office of Transport Department and approach the reception counter what service you want. He/she will give you a form and tell you which counter you should submit your application form.