Depends on the sensor...and your skills and devices.
And as far as throwing money at these cars....get used to it. Not a matter of 'if' that will occur... just a matter of 'if' you can keep it under some sort of control according to your definition of how much is too much
I've yet to find a way to detect a faulty (as opposed to outright dead) cam sensor off the car, as an example, but the ECT (engine coolant temp sensor - not the sender for the gauge) can be tested reliably based on info at this link.
http://www.fordfuelinjection.com/index.php?p=28
Some components require devices that were originally only available at the dealer and even where some are known to exist today, they're otherwise long out of circulation for the average owner.
These cars have complex systems, making it difficult at best to reliably test unit-by-unit (off the car, etc). Many times the issue is a combination of events, such as fuel grade, ambient temps, coolant temps etc. coupled with ground path reliability and conditions that exist only when installed. And then you have the need for wiring diagrams.....
The point is...while testing components is far better than simply jacking up the radiator cap and driving a new car underneath it, consistent troubleshooting without wasting time/money can require considerable time spent under the hood and elsewhere on an SC before an owner comes to grips with all of the details surrounding individual parts and how the car works as a whole. Good luck in any case