Scientific [GPS][1] and [SLR][2] have been used for [some time][3] now, and the [measurements][4] are rather accurate. Not only do we measure horizontal movements of tectonic plates, but also uplift as e.g. in the Tibetan and Colorado plateaus. 

Before the GPS, [paleomagnetism][5] were used in some studies to estimate the velocities; The age of oceanic crust are estimated by various methods and the travel distance is estimated from the width of the magnetic polarized bands in the ocean floor. To understand [paleogeography][6] and make good models we need to incorporate all branches of geology. E.g. [fossil records][7] are very important and [radiometric age estimation][8] of rocks. 

Some active faults (E.g. San Andreas) can also be directly measured with laser ranging or, in some cases, with a measurement tape to measure deformation of trenches or railway tracks. 

Still, scientists are trying to understand the exact driving forces behind plate tectonics. [Modelling][9] of [mantel dynamics][10] is an interesting field of research and many questions remains. 


  [1]: http://www.pocketgpsworld.com/howgpsworks.php
  [2]: https://en.wikipedia.org/wiki/Satellite_laser_ranging
  [3]: http://onlinelibrary.wiley.com/doi/10.1029/2003JB002944/full
  [4]: http://onlinelibrary.wiley.com/doi/10.1029/97JB00514/pdf
  [5]: https://en.wikipedia.org/wiki/Paleomagnetism
  [6]: http://www.scotese.com/earth.htm
  [7]: https://en.wikipedia.org/wiki/Biostratigraphy
  [8]: https://en.wikipedia.org/wiki/Radiometric_dating
  [9]: http://www.earthbyte.org/Resources/Pdf/Ghosh-Holt-2012-Science-PlateMotionsStressesGlobalDynamicModels.pdf
  [10]: https://en.wikipedia.org/wiki/Mantle_convection