agency web dev's : how do you create & test responsive design

I'm interested in agency web dev's in particular as they seem to have tight time budgets.

The speicific things I'm interested in are

• do you only try and make your design look good in a selection of breakpoints? if so what are they?
• how do you go about responsive testing, any tools or is it all manual?
• how to do you handle device testing?
• do you find methodologies like bem etc a time saver or reducer in regressions?