What year are you looking at?  The wide track tractors were introduced in '97 with some updates made until the '99 model year when several changes were made and anything from a '99 to the end of production were all the same with all the updates.  I've been looking at them as well and I'll try to list the updates I can think of.

1.  The midwheels on the undercarriage were changed from a one piece spoked design to a two piece bolt together, Cat wants to sell the updated two peice design to replace the spoked but there is at least one company who can retread the spoked wheels so it would be a nice update to look for but not a huge deal.

2.  The track alignment adjustment (right behind the front idler at the bottom of the frame) was changed from an allen wrench style bolt to a regular bolt head.

3.  The rubber on the drive wheel was changed from a diamond tread to a chevron tread to clean out better in muddy conditions.  

4.  There was an updated fuel tank, the original tanks were very prone to cracking and the updated tanks have two triangle shapes stamped into each side around where the lights mount, it's a $3,000 part from Cat so it would be a big plus to me.  

For the '99 model year they made some bigger changes including but probably not limited to:

The hydrualic pump was increased from 32 gpm to 43 gpm and the couplers were improved. 

The drawbar was changed to use a roller to swing on and the 3pt arms were changed to one piece cast arms.   

I think that is also the same time they went from the smaller suitcase weights and the belly weights to the big suitcase weights like what was used on the big frame E series Cats.  

I've spent alot of time talking to our local Cat ag mechanic and even looked at a neighbors tractor with him so I'd know what to look for and he said in general on those tractors you can get a good idea of the overall condition of the tractor by looking at the grille (if it's not cracked it's been taken care of), the tracks and the undercarriage.  If the sides of the guide blocks on the tracks are pitted the alignment is off but if they are pitted and glazed over they have "healed" themselves and it's not something to worry about.  Cat says new tracks can take up to 400 hours to get broke in and sometimes a new track will wear on the guide blocks like that until they get seated but when that happens they will get glazed over if that makes any sense.  At around 5,000 hours those engines will tend to need injector work but this guy said they have noticed that tractors using Cat fuel filters tend to go quite a bit longer before needing work than tractors using aftermarket filters.  If the serial number decal is missing off the frame it is also stamped into the frame ahead of the radiator where you can see it when you raise the hood. 

This pictures show the two piece bolted together midwheels.

These are the older style spoked wheels.  In this picture you can also kind of see that it has the newer style fuel tank, if you look close above the lights on the side of the tractor you can barely see the outline of where the updated tank is stamped for extra strength.