pcfernandesjr, I test at multiple levels.
First, I get the basics of everything done and I test in a similar browser as the device because I feel that is the quickest and easiest to do. I will manually resize the browser window so that it is in the same size and aspect ratio as the device.
Then, once I'm sure there are no bugs in any browser aspect size and ratio, I quickly do an export to the platform of choice (typically Win8 and WP for me) so I can get a good feel for the final results. A lot of times I just use the Visual Studio emulators, but running on the actual devices is best.
The export+opening in Visual Studios shouldn't take you more than a minute to do. To speed that process up you can skip the "PNG recompression" part of the export process by selecting None, as opposed to Standard.
Btw, this link could help you speed up the process too if you just want to test the regular browsers on different devices: How-to-preview-on-a-local-network