Factory automation for large builds

I want to build an automated factory of factories that produce 'one' thing.... Imagine 50 different factory modules working together to make one ship. e.g. fert to standard hardener to standard armor, and so on... All pre-set to make the correct amount of stuff to build from the final blueprint.

I want to push one activation button that turns on all the factories (can do this now) with maximum capacities on each factory already set to produce 'x' number of blocks. When finished a cargo block can suck the products into storage where I can then build the 'thing'...

Here's the tricky part, I don't want to have to go around to each factory to see if it's finished... If one factory only produces 1 block and another produces 1000 blocks - how can I know when they are 'all' finished? Can you put a sensor of some sort on a factory to turn off a light or something?

Gratitude.