The servo system utilizes encoders to report the servo motor position. External encoders that are placed on the telescope's two main axes can be added with a slave card. This gives the ability to detect drive slippage, observers pushing the scope out of position, and so forth.
For the stepper system, David Lane, author of The Earth Centered Universe (ECU), has designed an inexpensive serial interface for quadrature encoders. Called the MicroGuider 5 (MG5), complete details can be found at http://www.nova-astro.com/. Cost for the interface (ready to use), not including the encoders, is about $75US (see the parts webpage for the encoders).
Dave Ek's box is a small very fast reading unit
Finally, you can use the encoders from a mouse. Make sure that acceleration and multiplication settings are turned off via the mouse driver. Use the timer method of halfstepping instead of the delay method that temporarily turns off interrupts.