Video 177
This video is an extension to an older one (no. 158) about detecting change.
I detect change geometry and attribute within a form but this is done from an event.

The events are picked up by "$editcontext.editType" which can have the following values:

INSERT
UPDATE
DELETE
NA

So if you wanted to know that the user is updating an existing record, then in your Arcade you could check for that with:

If ($editcontext.editType == "UPDATE") { do stuff here}

or... is this a new record being entered?

If ($editcontext.editType == "INSERT") { do stuff here}

I use the above to calculate other fields on the same form.
You can of course keep track of changes anyway as a service property, but this lets you detect the event itself and do something from then on in your workflow.

Note this needs lots of testing in your environment! Field Maps for example will need some additional script to fully work but the above will get you started. Also, I would tidy up the logic before productionising this.