I was thinking of building a box containing a 12v power supply along with 2 12v light bulbs and 2 momentary test switches and a power on switch.
( or 2 posts where I could connect my 12v emergency starter battery thingey )
I would then source a set of RCM connectors from a junk yard ( need A,B & E ).
Plug up the RCM, turn on the power then press each test switch, if all is OK for that relay ( coil energizes, contacts close good, you get a light, otherwise a problem )
No probe slippage factor.

12v ( thru 15 amp fuse ) to connector B1 & B4
ground to both momentary switches and the 2 bulbs
B3 to one bulb ( fuel relay output )
A3 to one bulb ( engine relay output )
E3 to one switch ( fuel relay test )
B5 to other switch ( engine relay test )

You could expand this further and include the other 3 relays ( traction, horn and air ), but they do not effect the car starting.
Matter of fact, it's only the engine relay that does that, if the fuel relay is bad, at least the car will TRY to start.