Since you are a welder/fabricator, You should be able to judge the quality of the build. The first thing I would look for is just how dirty and nasty the thing is. If its pretty dirty, chances are it has some leaks. Finding the leaks on a nasty machine is usually pretty easy, look for the clean spots, that's where the leaking oil will have washed off the grime. Engine and type of pump is next. Will the engine crank easy and run and does it run or bogg when working. Bogging could mean the pump pressure is set to high or the engine is worn out or the pump is to large for the engine. Is the pump a two stage or single stage. A two stage will take less hp to pull, but if the engine will pull it, a single stage pump could give faster cycle times. If the splitter is equipped with a pressure gauge, fire the engine and dead head the cyl and see how much pressure it will build. Listen to the engine and see at what pressure the engine starts to bog down or die. Do you hear a squeal as the pressure bypasses the relief and the engine just keeps on chugging, if so, chances are you can turn the relief pressure up a bit for a little more tonnage. If the engine boggs way down or dies, the relief might be set for to high a pressure. If the build look like it will hold together, is leak free and runs, but doesn't have a pressure gauge, best thing to do is split a little wood with it and determine if it will actually do the work you want it to. If it wont split like you want it to, move on and look for something that will.