Two main ways to do it:
You could do a one of measure with stop watch and use this as a baseline
or
You could use a paper (or IT) system and ask your shift teams to record what they are doing at all times in the day
this is sometimes called loss accounting / loss management and is often used with the OEE measure.
If you want to know more, reply to this thread.
One method that is is is with a pager like device that randomly goes off ~20 time per 8 hour shift. When it goes off the user make down on a sheeet of paper what he was doing, i,e,; value-added, non-value-added, rework etc.
I belive Conway Quality system sells these pager type devices.
Productivity:
Productivity rate estimation can be done by using the stop-watch and making note of the time taken for producing a particular item taking into consideration the process parameters .A team of people can gather the readings to tabulate and plotting a graph between time vs job the optimal time taken for the production activity can be determined.This technique is called "time study".
further by applying "motion study" techniques the work simplicification of the production activity is further enhanced.
