Here's a good test to try in the store. You can do it in OneNote.
Using a business card as a ruler, try to draw a diagonal line, slowly (a few seconds to cross the card). Unless they have fixed it recently, the Surface Pro 3 cannot draw straight lines in this way, and they will be quite jagged.
In practice, you might never draw lines in this way, but the same jagged features will pop up everywhere, especially if you draw fine shapes carefully (as opposed to large strokes sloppily).
