單憑注冊號碼怎樣証明他是注冊結構工程師

see the Structural Engineers' Register under Buildings Department (BD)

http://www.bd.gov.hk/english/inform/e_rse_1.html
http://www.bd.gov.hk/english/inform/e_rse_2.html
http://www.bd.gov.hk/english/inform/e_rse_3.html

2010-03-05 13:34:51 補充：
if the no. is RPxxxxx, he should only be a Registered Professional Engineer (RPE) under a specific discipline (Civil / Geotechnical / etc)

You could search him at the following website
http://www.erb.org.hk/search.htm

2010-03-05 13:35:19 補充：
The duties of RPE and RSE are different