How is productivity measured in the kinds of jobs that can be done from home?

For manufacturing it's easy - just measure the output per day and number of working hours spent. But how is it done for finance, marketing, software development, creative work and such where the output is either limited (you don't need more financial reporting than last month) or is entirely different in scope and complexity every time?

If productivity could be objectively measured it seems to me it would be easy to determine objectively whether WFH is better or worse, but the discussion seems to be mostly based on personal opinions. Some feel that workers must be slacking off at home, some feel that they get much more done from home.