People have done this a number of different ways in the past. The easiest is probably just a “canary“ principle. choose a device that you don’t use for any other purpose. As long as it comes on after a power outage AND it tells the smartthings hub that it has come on, you can use that as your “canary.”
(a reference to “canary in a coal mine,“ meaning something that alerts you to an environmental condition). if you have a smart bulb which Will tell the hub when it turns on after a power outage (some do, some don’t) then you can use that bulb coming on to tell you that there has been a power outage at your house. And having done that you can then reset all your other bulbs however you want them to be.
There is a smart app that does this, but of course it’s groovy, and the whole idea would have to be rewritten once the transition to the new platform is complete. But if you look at that thread you can see a discussion of some of the devices that do it
AnD it’s the same general idea for any device. Find something which turns on after a power outage, and then write a routine based on that device coming on. This could actually be pretty simple, it could even be a smart plug. As long as it’s something that comes on to full power after a power outage and notifies the hub that it’s on and that you don’t use for any other purpose.
Another alternative is a device which can switch between battery and Mains power and notify the hub of the change. These are more often used to get an alert when power is lost, but you can also trigger on power being restored after an outage, it’s just that the logic gets a little trickier.
If you are in the US, here’s one example of this kind of device.
https://www.thesmartesthouse.com/products/diy-smart-power-outage-monitoring-kit
It’s true that if you have a hue bridge with hue bulbs, that’s by far the easiest since it’s handled in the bridge and the lights won’t even come on at all. With the other two methods I mentioned the lights will come on and then go off as soon as the routine runs. So it depends on exactly what you’re trying to accomplish.