Q: Smoking Msia Ciggs In Sg? (legally?)

hi just a random question..
as we all know in SG all ciggs are printed with 'SDPC'
so if any1 caught smoking a cigg without 'SDPC' = fined
but common knowledge is that for any1 who goes JB
ur allowed to bring back 1 pre-opened pack of msia cigg
when clearing the customs.. so my question here is...

if u went home after going to msia, u happy happy smoking
the msia cigg u brought back from custom, some fellow come
spot check u and say ur cigg is contraband and not duty-paid..

how do u prove that ur cigg is brought back from msia (legally from customs)
and not bought from some chiong seller or importer locally?

sry non smoker here so jus curious.. :o