It is advisable to have a peek inside first to make sure it's not full of bugs, dead rodents, pee, water, metal filings, and other such things but assuming it passes the quick visual exam, plug it in, flip the switch, and be ready to kill the power if you see smoke or sparks.

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I love finding electronics full of pee.

My stash of Pentium 3/4 motherboards were in a shipping crate in the basement. The resident border collie here kept having accidents in the living room. eventually they did enough of a number on the floorboards and the pee made its way into the basement, right into that crate of Pentium boards.

When it was too late to realize, I went in there looking for some blank RDRAM sticks, and voila, everything was ruined. Now that dog lives in the kitchen (which there is nothing underground under, and also has linoleum flooring...)

Although this site has its horror stories to scare me otherwise, I always just inspect and power up as well.