If re-assigning a settler means that the previous assignment will stop, it's a problem since it's not clear whether a settler is already assigned or not!

They can be assigned to more than one thing. They can also be assigned to a bed, and that's not stated anywhere.

Go to workbench then to food/water i.e. resources.

Then go to misc.

Choose the "bell".

Place the bell.

hit escape.

Activate the bell.

Then walk up to each settler, and they'll give you hints on what they are doing.

they might say something like "the crops are coming in pretty good"

or they might say "the farm aint much, but its something".

This means they are set to farming.

Another way of finding who are NOT assigned.

The game explains that any new settler which is NOT assigned to a spesific resource, will be scavenging for resources you need.

So you will often see them just standing around, or working on one of the crafting stations.

If a settler is using a armor crafting station, cooking station, weapon crafting station etc.

This means they are scavengers and you can then assign them to other jobs.

Another way is to make a large cluster of cheap chairs.

( i prefer the red comfy chair my self, because it only requires wood and cloth).

Place a bunch of chairs and who ever has "little" to do, will sit in one of the chairs quite often.

(Note: At late evening or night, all settlers will look for a chair to sit in, so dont be picking anyone from a chair at that hour, because they are resting before they go to bed).

The rest i bet u can figure out your self

Some people give varying hats to settlers performing similar jobs. I give them some piece of armor that I've changed the name describing their job. So when I meet a settler without a hat or without a descriptor armor, I know he/she is unassigned, and I can look at their inventory to remind me what I assigned earlier.

There is a loading screen hint that says we can See what they are assigned too by highlighting them, but if it works I can't figure it out so guessing a bug.

You can see what they are doing by entering build mode and target them, the stations/resources they are assigned to will glow green.

Hover if station/resource is not in LOS then its a bit harder.

this is the best solution so far, or indeed giving them certain clothes, but with big bases it might be a hassle but you should always be able to work your way around.

Yes this is true, but when the base is swarming like an ants nest, and they keep swapping places, or similar, it will be impossible to keep track of who is doing what.

In addition to this highlighting, when targeted in build mode, they should have a little Label tag floating above their head, which says what resource they are assigned to.

Like "Carrots" or "Muttfruit"

Then those who are not yet assigned "Scavenger".

and those who are completely fresh who just walked onto the settlement "Unassigned".

Thats what i think could be a great improvement.