Well, I don't go through a "checklist".
I look for overall condition (taking in account it is a used tractor). I make sure it starts easily and all controls work properly...including the loader (if it has one) and 3PH. Easy check on these are with an implement attached to the 3PH. I lift the front end with the loader and run it to the limits of travel...as long as there is no squealing, and both loader and 3PH lift OK, there's probably no problems.
The tractor should pull strongly in a lower gear or range. Shifting (gear guy here) should be smooth with no grinding or slippage.
The tires and paint should match the hours showing on the meter unless it's been repainted and the tires have been replaced.
All instruments should operate and so should the lights.
I also look at the seat and floor. If the seat is torn up and the floor shows wear beyond the hours on the meter...well, that could be a concern.
Most importantly, know the dealer's reputation. Check their rating by the Better Business Bureau. I've bought three tractor in the last 10 or 11 years. Two were from the same dealer who has a good reputation, but is a bit on the expensive side. The other was the most risky being an internet and out of state purchase. As far as the out of state purchase, I figured I could refuse delivery if it did not meet my expectations. this was the 790 which I kept 6 years.
Definitely read about the tractor before going to the dealer. You'll pick up good info as far as potential weaknesses to focus on.
I doubt most dealers will allow you to run a compression or leak down test. Those items you listed in your post (Smoke colours, seals leaking, rad cap checks, head gasket edges, starter noise,ring gear noise, belts, alternator output under load) are visual or aural (except for the alternator output)...so look and listen.
Buying used always entails certain risks. Just pay more attention to the tractor then the salesman.
BTW, when I looked at the 4400 I just bought, the salesman showed me the controls and started the tractor. After that, I was on my own and no pressure (he was with another customer..just observed me enough to ensure I did nothing stupid).
Hope this helps...