to people who know what the hell they are doing, is there a site like this?

http://www.whatismyscreenresolution.net/multi-screen-test?site-url=http://www.kotakuinaction2.win&w=540&h=960

Or a browser plugin or desktop app something where I can test locally hosted files?

I can always try on two browsers, one desktop sized and one roughly mobile sized but again if someone who isn't a caveman at web stuff has advice let me know