Hi,
Oh too bad, i missed the original title
Someone fill me in?
You have to be careful when testing pots because they are not always rated for power based on their resistance element, but rather on their contact mechanism. This means the only real way to tell is to read the manufacturers data sheet like Ron pointed out.
If you dont know what it is and it is a 'normal' size pot, you can probably get 1/4 watt out of it or even 1/2 watt, but only some pots can go higher than that.
Aside from the wattage rating, there is another rating that is quite important too. If you intend to use the pot in a typical 'volume' control type application, you'll also have to find the 'maximum number of rotations' specification (actual name of this spec varies). Some pots can last for a large number of rotations, while others are limited to as few as 50 complete rotations back and forth. That means the pot could wear out quite quickly if it's not rated for continuous adjustment. Something to keep in mind if you dont know the original of the junk box pot.