analog inputs on each side of the relays and make a comparison between In and Out. Can anyone see any problems with that
Yes, if you are talking about the coil it doesn't tell you much at all, and if you are talking about the contacts you would never read the voltage drop as it will be uV probably.
It will depend on exactly what you are switching, using a spare contact pair is the easiest way but that's still not 100% as the other contacts may have a fault.
What are you switching?
Rob