The onus is on the applicant to prove seven years of residency. I believe these days ImmD need to see tax returns, rental receipts, leases, etc. ImmD can also check whether the applicant has actually stayed in Hong Kong or has had very long absences (2-3 months okay, 6-12 months not, stint in prison not). And then there is the declaration; I suppose a person could cross their fingers or something.