Depends on a couple things:
1) How does the motor sound? Any skips, wobbles or grinding?
2) How sharp is the blade?
3) Does it have a useable sharpening system on it that locks in place and with useable sharpening wheels, and can the be replaced? (Many times you'll see a slicer and there's sharpening stones with them; you buy it and get it home and the sharpener is for an entirely different model, it doesn't fit it because they lost the original one and couldn't find a replacement, so they stuck a random with it figuring you'd be none the wiser until you tried sharpening it. With no sharpening system and a dull blade, it's useless.)
4) Does it have frayed wiring?
5) Does it use 110VAC or 220VAC?
6) When you dial the thickness gage to 0 are the blade and fence flush? What is the gap between the blade and the fence? Should be no more than ¼" or so; too big a gap and it will yank the meat into it instead of slicing it.
7) Is there a push guide so you don't have to hold the meat itself so your fingers are protected, or is it broken in any way?
8) Any problems with the on-off switch?
9) Can it be disassembled easily and go back together correctly with no missing contact or hold-down points so it can be cleaned and reassembled properly and safely?
10) Turn it on with the fence set to 0 and check the roundness of the blade; does it go around equally or does it get closer and then further away from the fence as it goes around, indicating an unbalanced blade (sharpened incorrectly) or a failing motor (bearings giving way)?
Just some basics to check for.