There are 2 types of SIM locks. Those that can be checked online are those locked by Apple on behalf of operator. Malaysia iPhone all sim-unlock, so no need to check there if Malaysia unit.

Another type (specifically for Malaysia) is locked by MCMC, this lock only applicable to operator in Malaysia, they all work together to maintain one database to lock device based on IMEI - stolen, not paid installment etc.

So ask for receipt is the best way to overcome.