From my experience replacing backlights, I've come to the conclusion that it's a better idea to simply go with a brand new backlight each time instead of salvaging old ones. When you use a salvaged backlight, you don't know how much use it has already seen and how much life it might have left. When you use a new backlight, you know you've got the best odds of getting the longest life and maximum brightness out of your replacement.
Also, you're going through an extra 50% of labor just to get the old backlight out, and you might not even end up with a good one. I've found it well worth it to simply spend $10-20 on a new lamp each time a screen needs one.
Now, I do this because I'm repairing them for customers. I wouldn't want to take a risk by putting in a used backlight in a customer's computer. In a computer of your own, it's a different story. It's not as big of a problem if one goes out on your own computer that you know how to fix.
As far as testing a CCFL for electrical resistance, it wouldn't quite work that way. The leads on a fluorescent lamp aren't electrically connecting to each other while the lamp is off. It's only when the gas inside is ionized that there is a current running through and an electrical path existent between the two leads. Testing for resistance would have to be done with the lamp powered on, and this would most likely damage the meter you'd be using. An oscilloscope might be able to do this, but I wouldn't know for sure.
I would think a much easier way to find the resistance would be to find the driven voltage and measure the current draw, and then use Ohm's law to get the resistance of the lamp. The ammeter would still need to withstand the high voltage. Another problem with this is that you would need to know what the resistance is supposed to be in the first place to know how others compare.
All in all, I would think measuring the the light output with equipment or just your own eyes would be the best way. If you had a new LCD to power up and compare side-by-side to each test LCD, you'd have a consistent reference point to go by.
A dim CCFL is the usual cause of a dark screen image. The CCFL can be dim either because it is worn and can't put out as much light as before, or it could be cause by it's inverter being damaged/worn and unable to supply the lamp with enough power. The LCD panels themselves can degrade and grow dim, but it's pretty rare that you see anything more than just a slight yellowish or grayish tint.
I hope this helps!
That just might be my longest post ever.