Physical intrusion detection is the act of identifying threats to physical systems. In many cases physical intrusion detection systems act as prevention systems as well. Examples of Physical intrusion detections are:
There are also some more creative physical intrusion detection techniques that may be of use to people attending cryptoparties.
Just put nail polish with lot of glitter on the important screws, take some good pictures. Keep the pictueres and make sure of their integrity. Compare the nail polish with the pictures before powering on the laptop.