I use a browser that I don't log in with much (say Opera) and try from various locations, including through a proxy if I can.
I also use the rankings in Google Webmaster Tools, though some high rankings can be excluded for various reasons.
SpyFu and some others will give you other data.
That said, it has become nearly impossible to talk about a ranking for a keyword. Most people are having their results personalized in some way. Even if you've never logged into Google before, your IP will have a significant impact on what shows for many searches.
On a site I work on, we've recently seen a crash in traffic without any major reported or observed changes in rankings.