Judging from the Robot letter in the article, the easiest way to spot a robot-penned letter would be that specific words are all the same. Look at the word 'the' in the picture, or even the 'th' in 'this'. They're all identical.
The robot seems to be working from just a small pool of characters.
Some sort of randomisation algorithm would work well here, varying the size, weight and slant or jitter of each letter would at least make it appear more random, and harder to spot.
Having said that, if you are someone who works on this sort of invention, please take a long, hard look at what you're doing with your life and consider just how this invention is contributing to the advancement of mankind.