You need to judge each site's traffic on its own merits, there is no one quick way to determine whether traffic figures have been faked.
There are little red flags, though, like:
- ridiculously low average time on page
- the same analytics code displayed on multiple sites
- URLs that have been visited that don't exist on the site
- weird search phrases you wouldn't associate with the site
- strange locations of visitors you wouldn't expect
I use Google Analytics so the above points are related to that package. If the site you're interested in uses GA, ask for guest access so you can explore the stats yourself.
Also consider making a back-up like:
I will buy this site because the name is good and if it is not as advertised I will use to ... So the purchase would be ( if not as advertised ): was quite expensive but I still had ways to use it.
Google Analytics Guest access is by far the best way to check statistics of any site and your chances of getting ripped off would be lowered to extreme heights... Proxy & Bot hits are what makes most of the FAKE traffic & its somehow difficult to guess without poking around into various Analytic areas.