How to know if used iphone 4 factory unlocked?

hello guys iam just new here. learning my way around the new iphone 4. Need some help from you guys.

i just recently bought iphone 4. the guy that i bought said its factory unlocked. it's on version 4.2.1 modem firmware 3.10.01 he let me check the phone its has no cydia or something. its bare stock. no application installed just the usual apps you can see on the spring board. he tried his t-mobile sim dialled and text some number it work. he also let me try my international roaming sim card it works also. i am just wondering may be he just software unlocked it. is there anyway to hide those cydia and ultrasnow, greepoison jailbreaking and unlocking software? i just want to make sure just in case i want to update the software it wont be locked.

one more thing when i entered the serial number ofthe iphone 4 i bought in the apple website. the message is "We're sorry, but this is a serial number for a product that has been replaced. Please check your information and re-enter your serial number. If your information is correct, you may need to contact us" what does it mean?

Thanks for your help guys..













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