I would make a distinction between two types of verifications:
One, that the column is sorted in a particular order;
Second, that there's an indication on the GUI that the column is sorted in a particular order.
The difference between the 2 types of verifications is that the one is direct while the second is indirect.
With the first approach, one would retrieve the column's contents to an array, copy it to another array, sort one or the other, and then compare them to see if they're equal. If the answer is True, then the column is sorted; otherwise it's unsorted. The particular order (ascending or descending) can also be verified this way.
With the second, which you're trying to use, the data order is not verified, but only the GUI indicator for a particular order - that is, you assume that it's sorted, and only ask whether it's ascending or descending.
The Bitmap Checkpoint, with which you're trying to implement the second type aforementioned, is a very problematic approach to verification. What I'd suggest is that you explore the the JavaTable object model and see whether in the class properties there's some indication for the change in the state of the object.
I hope this is useful.

Cheers!