How to test if a dishwasher's below the floor heating element is faulty?
Continuity is "An unbroken electrical path in an electronic circuit or connector[1]". First thing, unplug the dishwasher or if hard-wired turn off the circuit breaker. Unplug the connector to the heating element and then test the contacts on the heating element. 

A heating element is a semi-conductive (typically coil) that creates heat as it resists the flow of electricity through it, a.k.a. a resistor. Also, even though there may be a clear positive and negative fed into/through it, a resistor does not have positive and negative terminals so orientation of your testing leads doesn't matter. A conductivity test will provide some feedback, but a resistance test with provide more accurate feedback, set your multimeter to Ohms Ω and check that the value across the heating element is within manufacturer's standards.

Whenever testing a heating element, make sure you are actually testing the element and not a circuit before it that drives it!

[1] http://en.wikipedia.org/wiki/Continuity