It's tougher than it looks.
Especially so with newer Chinese made products actually adding lead weights inside their cheap plastic cases to make their cheap products feel as though they were cut from a heavier cloth.
Some rules of thumb that I have when it comes to buying old power tools are:
Does it look like something your grandfather might have owned?
Is it of all metal construction? If not, are the plastics of a high enough density to make them almost as indestructible as metal?
Does it plug directly into the wall? And does it work?
If you can answer those questions in the affirmative when it comes to any power tool, chances are it is of good quality.
The best way by far imho however is just to get out there and start looking.
After a few months, you'll begin to notice lots and lots of exactly the same items showing up in the same shelving sections.
The same kettles, the same breadmakers, the same drills, the same table lamps, most all of them having been made in the past 20 years and currently knocking on death's door.
Once you've seen enough of that crap, when a quality item displays itself in front of you, you'll know it's a great deal and snap it up with excitement.
I love thrifting.
Good luck out there!