I actually probed out the signal on an oscilloscope, which is how I know the signal's voltage and frequency (it was kind of weird, actually, I had always thought of LCD as being a DC thing)
I want to be able to have the microcontroller follow this signal though, without a human intervening.
HOWEVER, detecting the signal on an LCD is a really last-ditch method. I would try VERY HARD to find a more suitable method. Even if you manage to amplify the signal, rectification could very easily mask any slight change in the signal. I would not consider this to be a viable method at all.
I think it's pretty much the best choice in my case. I'm trying to follow the state of an external device, and the controller is pretty much a black box, with no external communication, but all necessary information gets displayed on a purpose-built LCD.