Depends on whether it has a plastic camshaft in it or not. The older engines have the plastic cam, the newer ones have a steel cam. I'm sure someone can give you a serial number break if you need it.
Personally my 445 has 2200 hours on it and still running strong! Good reliable tractors. I would say though if it allready has 1200 hours on it, it probably doesnt have the plastic cam, or has been updated.
Proper matenence and care will make all engines last longer, so if service records are available that's a plus. Also, if the guy seems like he treats his equipment well, I find the tractor has been taken care of well, and be more obligated to buy it.
Just my $.02