Two types of touchscreen technologies are available in market widely, capacitive and resistive touchscreen. The each is having own pros and cons.
To check whether a touchscreen is capacitive or resistive you need the device to be live. You can’t check it touching it dead, it must be in operative condition. A resistive touchscreen operates by resistances generated. So it can be operated by anything nails, finger, plastic, metal where as a capacitive touchscreen needs capacitance to be generated which needs conductive material. i.e. human skin.
So whenever you want to detect if a screen is capacitive or resistive just try to operate it by nail, resistive will give you response where as capacitive won’t. The benefit of capacitive screens I have seen so far is they response a bit faster than resistive touchscreens.
Laptop touchpads are capacitive devices. Samsung uses capacitive type touchscreens, recently used in Corby. Devices from apple use capacitive touchscreens. i.e. iPhone, ipad etc..
Nokia use resistive touchscreen in its touchscreen devices. 5800 will be a good example for it which is quite famous in market.
Another trick to detect the touchscreen type is resistive touchscreen device comes with a stylus where as capacitive touchscreen devices can’t as they need capacitance to be generated. In above image you can see nokia 5800 stylus.