First, look at the heat coil in the bottom of the tub to see if there is any visual damage, such as a melted or burned through spot. If you see a spot like that, the element is bad and needs replacing. Otherwise, the dishwasher needs to be pulled out so the heating element can be checked at its terminals on the bottom (unless you're lucky enough that the terminals are at one of the front corners, as they are on some units) outside of the tub with a multimeter set to ohms. Depending upon the manufacturer, the element should read 15 to 25 ohms of resistance. If it reads "0", then it's bad and needs replacing.