Lots of things you can check, but keep in mind the front axle has a trunnion pivot in the center.
Because of this pivot point, a bend (or a crack) in one of the long frame members can show up as a difference in the curb height measured from identical left/right locations on the front end of the frame.
A crack is the easiest to detect for obvious reasons, but if the section of frame plate is concealed by something covering it, even that might be hard to detect,
A bent spot could be mistaken for a "normal shape" unless there is some other damage (paint popping or showing rust streaks in paint fissures).

Check also, the mounting brackets that secure the transaxle to the frame. If one of those brackets has wear in the bolt holes, or other defect that might let one side settle lower than the other, it would cause the entire frame to tilt (axle pivot again) toward the sagging bracket.

A twist in the frame (frame assembly torsion) is the most difficult to detect and can really only be "inferred" by eliminating all the other possible causes first.

So, inspect the entire length of both long frame rails for cracks or bends.
Check the rear transaxle mounts for loose fasteners or other defects.
Check the front axle trunnion bearing and bolt holes in the axle mounting plates for wear or breaking of axle beam or the mounting plates where the axle pivots.

If you do not find any defective components at any of those areas, it would be a safe bet the frame may be twisted.

IS the frame welded togather or bolted? Some craftsmans LT have stamped steel frame pieces and are bolted togather in three or four point. if bolted maybe you plowing of snow and the loosening of the bolts may have twisted it out of alignment over the years. If it's welded the as suggested above. Basiclly your going to have to strip all the cowl and fenders off to see what you got, probably need good inspection anyway.

Poor detail, but enough to illustrate parts.

{{gwi:349878}}
{{gwi:349881}}

Great Responses guys.

yes- The frame is bolted. I checked all fasteners and looked for oblong thru-holes. Nothing too obvious.
In picture above look at item 44. This is where the mower deck front suspension attaches. This is where I measure the difference in height.

I have been drooling over a massey....maybe it is time. My yard warrants a welded frame anyway and I've pushed the GT to the limit (especially in the winter).