[gregjor]

You won't know what past employers say about you. If you can show a past employer badmouthed you or hindered you getting hired you probably have a legal cause of action in the US.

Past employers generally only confirm dates of employment, title, vague job responsibilities. Calling a company to check prior employment will go to HR. The most telling question: Would you hire Joe Bloggs again? Beyond that past employers rarely say anything for fear of lawsuits. A former "toxic boss" may cost you a job prospect but by doing so they have put a pot of gold on the table for you and your lawyers.

Only an idiot lists references when they aren't sure what the reference will say if contacted. Of course recruiters and employers know that, so they only give so much weight to what former colleagues or professors have to say.

Professional certifications may have some value, as long as everyone knows they are fair and objective and measure actual abilities. So many useless certificates exist, and even the good ones get gamed, that I doubt they carry much weight.