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.