I hope you are not getting those China made clone sets which looks 99% like the original
Difficult to advise you how to check whether is original unless you compare it side by side
You can look at the rom details and verify such rom exists at sammobile.com/firmware
Best is stay away from such dubious deals
Read the main thread sticky on how a buyer was cheated buying faked samsung sets

http://forum.lowyat.net/topic/2489387

bro, i advice you take precaution, never believe in rampasan set etc... seriously.... 

yeah cheap? how much cheap?
better deal is through 2nd hand if want jimat abit...
i scare you will be cheated, if you are advance android user you can check it through setting info, but
if you still new to android i advice you never accept this deal, you will regret it, 1st, no warranty... warranty is must!!

warranty 3 month but they will never follow the rule, if you want, buy 2nd from some user here who jump to note2 or in garage sale 

Best is get a friend who has a S3 and compared with it side by side
Next check the bootup and settings details
Test the S3 unique phone features - whether you have all those special features at settings
If you are unfamiliar with Samsung phones, better stay away
Buying such stuffs only for seasoned user

You still bought the phone or just checking it..?