Have a read through this thread for an app that handles this very well, and there are many side discussions of different approaches.
Washers are easy with something like the Zooz ZEN15 power monitor, assuming your washer is 120V. It takes some experimenting to set the right thresholds. Just because your usage drops to near 0 doesn't mean your laundry is done (it may be filling with water if a top loader, for example). Tweaking thresholds is what the app is good for. Also, don't use just any old energy monitoring plug. It needs to be rather heavy duty.
Dryers (electric) are typically 220V. There are some solutions discussed in the thread. Personally, I have Miss Alexa listen for appliance beeps and trigger an announcement (and flip a virtual switch) that the dryer is done.
Developers - Hubitat
