I want to be able to trigger things based on whether or not a 120v standard outlet has been turned on or off. Like something that would plug in and just detect power and report on it. There are lots of smart outlets, but I need to see the status of a dumb outlet.
What device can I use for this?
If the dumb outlet is turned on/off by a switch, you can replace the switch with a smart switch and monitor the status of that switch without setting up anything to control it.
If you mean you want to know whether a device plugged into that outlet has been turned on or off, you could install a smart outlet with power monitoring capability, leave it turned on at all times, and detect on/off based on power consumption.
Sorry, I’m confused about what you’re asking for.
A typical dumb outlet is never “turned off.” It always has power available unless there is a line outage. Devices plugged into it can be turned on or off, which means there is a “draw” or not, but the outlet itself is always on.
The exception is a “switched outlet,” where there is a switch on the wall which can cut power to the outlet.
So are you asking for:
A) notification if there is a power outage on the line, such as in a storm?
B) notification if something plugged into the outlet like a tv is drawing power (tv is on) or not (tv is off)?
C) notification if a switched outlet has been turned off with the wall switch?
There are potential solutions for all of these, but they are not the same solutions. 
The dumb outlet is on a switch. I want to be able to trigger based on when the switch is flipped on, but prefer not to replace the switch (it is a 4-way). So I want to sense whether 120v is present or not.
There are several different ways to do this, but I’m not feeling well today and I’m going to have to leave it to other people to go into most of the details.
However, by far the simplest solution would be to put a smart switch cover over the existing wall switch which physically moves the dumb switch underneath. That switch cover can report its on/off status to smartthings and you won’t have to change any wiring in the four-way or the outlet. 
There are two that work well with smartthings. Both are battery operated devices that fit over the existing switch and physically move it. Both are also on the official “works with smartthings“ list, which is a plus since they should work straight out of the box.
The ecolink is a Z wave device. Prices vary a lot, so shop around. There is one model that fits over toggle switches and a different model that fits over rocker switches so check to make sure you are getting the right one.
https://www.amazon.com/Ecolink-Z-Wave-Automated-Lighting-TLS-ZWAVE5/dp/B0716YVRCV
https://www.amazon.com/Automation-Lighting-Switch-Ecolink-Control/dp/B07DCK8H8R
The Third Reality is a zigbee device. It comes with an adapter so the same model can work with either a rocker or a toggle switch underneath.
https://www.amazon.com/gp/product/B07K3TRG6W/
Neither of these are particularly aesthetic choices, but they work well and solve retrofit problems where you don’t want to do any wiring.
We should also note, though, that both are fairly noisy, about the same as a smart lock, because they are physically moving the device underneath. So that’s another consideration for this option.
Check this thread out:
