lol Benji are you gunning for a spotter's fee?!
All good mate. In terms of businesses' reputations, the easiest way to find out is to Google - don't bother looking at the happy customers with glowing referrals listed on their websites, it's how they deal with issues that are the true test of how good they are.
If you're looking at dealers, ask for either 1) the auction sheet or 2) the export certificate as evidence that the kms are legit. If they try and tell you that they didn't buy it from auction, then tread VERY carefully, as I could count on one hand the number of private sales done in Japan by overseas buyers. Auctions are far cheaper, and car dealers have never met a dollar they didn't like.
If you think the export certificate is photoshopped, (and some dealers are getting pretty good at it these days), then run it through JPEGSnoop or a similar program to see if it has been edited. If you still have concerns, then there are ways of finding out the correct export certificate readings in Japan (PM me for more info).