you can use a test light, using the battery power of the truck and gate them to see if they pass the gate test....
you will need to disconnect all the connections to the scr to do this effectivly, but if your going that far you might as well remove the component and isolate it away from the control panel.

however...
the best way to test them is to use a 'handyman' component tester
the handyman can gate and also leak test the scr's