Tell him that its has an in line converter and the dials read in MPH but the dials are marked in KPH. - bet you he wont even driver the car fast enough in and out of the garage for the needle to even move. let alone tell that its not right.
If you jack the front of your car up and turn a front wheel torwards, with an LSD the other wheel will turn forwardsn too. without an lsd, the other will turn backwards. thats one way of checking
Its worth having a look, just keep your whits about you and if its too good to be true... it probably is. I mean there is no harm in having a look. Just dont take any money with you so you dont put a deposit down spare of the moment thing