Listen for any faint crackling noises, which hopefully will be nothing more than noisy pots.
Also...after the unit warms up, I always smell each receiver for a tobacco/nicotine smell which usually indicates the previous owner(s) were smokers. I can't stand that smell and had to pass up a nice Pioneer receiver because of it.
*If* you can stand that smell up close for say a day - usually, most of the fine dust/debris can be cleaned off, but it is lots of (gruesome) work. Then it has eventually chance for the smell to eventually subside to an acceptable level.
I had to endure my own above medicine once with the Yamaha CRW-F1 cd burner (the best of its ilk- ever) - I felt like I was cleaning previous owner's ashtray for about a year's worth of "spent cigarettes" (English not my first language). The damn thing was so smoked that the only course to get the lens (and everything else...) clean so that it would read the CD-R was to use Q-tips (LOTS of them...) moistened with alcohol .
It is still happily burning CD-Rs....