There is a Web Developer add-on to firefox, that I use to view my sites. It will render a page with multiple views, as:
Mobile Portrait (320x480)
Mobile Landscape (480x320)
Small Tablet Portrait (600x800)
Small Tablet Landscape (800x600)
Tablet Portrait (768x1024)
Tablet Landscape (1024x768)
I don't know about the accuracy, but those are the boundaries I use for media queries.