Thank you for the replies!
I jumped the pressure switch, and nothing happened. Couldn't tell you if the relay(s) clicked, since the engine was running.
I know for sure that the high speed fan works, as I could clearly hear it running earlier today.
I will check the other gadgets that Willy pointed out.
I do have an old fashioned meter to do some testing with.
Since I know that at least one relay is working, I suppose I could swap them with each other, to see if the fan comes on, once I jump the pressure switch. Agreed?