electrical cooling fan question ?

Hi , is there any realy for elecrical radiator colling fan ? where
is it located ? i was told when a/c is turnedon the electrical
fan in front of the radiator should turn on, also when when
the engine gets hot. right ? on my car it won't turn on , however
when remove the plug to the radiator fan switch and jump the
wires it will comes on , is there any way to test the senser(fan
switch) ? any idia please let me know . thanks fred740il