GreenTech covered it pretty good. The augers should be a tell-tale indicator as to what the machine has been doing, unless they're fresh & all changed out...and then I would think, for sure, that the machine had been in more than its' fair share of corn.
I don't harvest any corn myself, but have several friends that do, and it amazes me how fast augers are eaten up in corn. I realize that the sheer volume is one factor, but the material itself is certainly another. and yeah, I've heard that soybeans are even worse (but NONE of those are grown around here
Look at overall condition, too. Try and see the machine in a natural state, if possible, instead of after it is detailed. I recently looked at another used combine I was/am considering. I got to see it right when the guy brought it in---before
they had a chance to detail the combine, and that was a big advantage for me. I found a couple of leaks that I'm sure I would not have been able to locate after
the wash boy had cleaned 'er up.