In some cases you can tell this directly. A task that increases the food supply, like foraging or gathering the harvest, is only complete after the villager has reached its destination and gone through the graphic of unloading - you can tell this because the food supply figure isn't updated until that point. It seems you can then pick the villager up and take it back to the berry bush or other place of work.

If you pick up a villager who is carrying a basket of fruit or a bowl of water, the item falls to the ground and disappears.

Breeding might be an exception. The population figure updates as soon as the couple finish their embrace, so you might be able to move one of them away before they 'go indoors' - but that seems cruel!

I'd guess that for tasks like clearing the beach, you can pick the villager up after they've dropped the piece of debris.