We normally check our site SEO specific keyword positions by using the Google Adwords Preview tool by setting geo location as USA and preferred extension as .com .
We also verify the keyword positions using the "Google Global" addon as well..but from last couple of months there is LOTS of difference in keyword positions to both Preview tool , addon and google.com with &gl=us.
For Ex: a 2-word phrase for my site in preview tool shows in 6th / 7th position and where as in google global and google.com shows as 4th position.
We see similar results for related phrases.
What is the best practice to check the SERP report for a website. Please advice us
[edited by: tedster at 4:23 pm (utc) on Aug 6, 2012]