If you want to see how your keywords appear in the US version of google.com, you can sign out of google, clear cookies / disk cache / whatever and enter this URL:
http://www.google.com/search?q=QUERY&gl=us&pws=0 I keep a separate "clean" profile in firefox where cookies and whatnot have never been enabled and I use it only to check rankings.
I'd advise against using a software to check your ranking.
It uses your IP address, and two cases can happen :
I suggest using a SaaS tool, like myposeo.com, as it'll use its own IP pool and doesn't take your computer's data, but theirs.
Hope this helps,
There is no site google.us. Google owns it and redirect visitors to main site google.com
Use software to check rankings... To be able to select which Google datacenter you are checking you need to use a tool like AWR (AdvancedWebRanking).
This will let you enter a list of keywords you want to track, the search engines you want to monitor and specifically which datacenters you want to query (e.g. Google US). Furthermore it saves your results so you can trend the data over time.
Subnote: everybody does it.
Agree with edralphs comment - software / SaaS is the best way to do this.
Another alternative - seomoz's tools. Less chance of getting your ip banned with local tools - allows you to monitor multiple competitor sites over time, and provides good feedback on what changes you should consider.
Not cheap, but good.