I can see bees bringing pollen into my hives, but how do I know if a honey flow is on?

I don't think I can simply assume where there is pollen there is nectar. Or can I?

If I frequently go into my hives I can note cells being filled, but is there a way of observing from outside the hive that there is a flow on.

Keeping is a hobby, or at best avocation. Just noting variation in activity levels is going to be a marginal method.