that's something I don't understand. the Dev tools in Chrome and Firefox make it really easy to test through every resolution without problems. every UI change I make I view in all possible resolutions and its quite obvious of something breaks and I adjust accordingly.
If it takes too much time, you are probably doing something wrong like having hardcoded layouts for different resolutions instead of using reflow as much as possible.
Even with all reflow it's hard. Should I reduce a width a little bit more so that it become 3 columns instead of 2 on iPad vertical at risk of having some line wrapped? Things like that.
For my day job working on a UK e-commerce site that gets roughly 100,000 uniques a day, our split is 70/30 for mobile/desktop. Unfortunately within the company it’s closer to 1/99. So no matter how many times I repeat the phrase “mobile first”, the higher-ups just don’t get it. Luckily I can make rational decisions for them!
I like to start on the desktop but build in a way I know the objects will flow. Then I set up a tester with a bunch of breakpoints and check as I go and make micro adjustments to each element that is "off". Then you can set the element inspector to the side and just keep shrinking the screen to make sure nothing is awry as the width gets less and less. I don't really have X sets of 'rules', just elements adjusting at the point it feels right to adjust them.
I learned this lesson the hard way. The client was on desktop. The users were 40% phones. Everything was great for months and months.
Then one day the owner of the company that owned the company tried to look at the web site on her grandkid's iPad. Fan met shit that day.