Isolate the supply at the switch remove front cover , Check continuity of cpc from the DP solator switch , check continuity from the Live to neutral ,then across each element ( normally have to disconnect each element to do this ) insulation tests earth to neutral live to earth but omit the neutral to live as this will give you 0.00 , the resetable thermal cut outs check to see if tripped ,will show open circuit if it has , reset if required , the stat is a difficult one cannot realy be tested ,but if suspect replace if no continuity and the thermal trips are ok check the elements ,to inspect the elements remove inner cover remove bricks check elments for distortion breaks holes etc , normal fault blown element runs cooler than normal as elements are wired in parallel