The field current on a ST-5 at full load is not all that great, that bridge should handle it just fine. bolted to a metal case/box it should be bulletproof:) The hands on test is a good gauge of heat. 120F is about the highest temp the human skin can sustain without sustaining permanent damage. Above that, the time you can maintain contact is measured in seconds. Personally if i can stand it for about 4-5 seconds it is around 150F
As for testing a bridge rectifyer, it has 4 terminals. There is a diode connected between each of the 4 leads(4 diodes). With a volt/ohm meter, you measure between any two leads(1-2, 2-3, 3-4, 4-1) then reverse the leads and repeate the measurements. Each terminal pair should have low resistance in one direction(meter forward biasing the diode) and high resistance measured in the opposite direction/lead polarity(meter reverse biasing the diode)...