Newer power tools are double insulated. The wiring, motor, and switch are insulated. The case is also made of a non-conductive material such as plastic. Many of these newer tools do not need a three conductor plug. They are much safer to use, especially in damp conditions.

Do not use an older metal case tool if the plug only has two prongs. A three conductor plug can be added as a replacement. The third conductor (green) is attached to the case of the tool with a screw. The ground screw is usually colored green. The ground conductor can be tested without disassembling the tool. A multimeter, set to measure resistance, can be connected between the ground prong of the plug and the case of the tool. The resistance should be very low. It is a good idea to use a ground fault protected receptacle with a single insulated tool. A GFCI should be used when any tool is used in a damp location. Check out my GFCI article on my blog.