Nokia 5110 Screen Pixel by Pixel
The 5110 Nokia LCD display is based on PCD8544. It is not possible to read the device display memory. The controller has only serial data input. 

Typically a graphical device driver for that display will use an offscreen bitmap (buffer). To check the pixel status you would need to check the status in the offscreen bitmap.

Some device drivers use only text and copy font bitmap directly to the display memory. In that case you cannot check the status of the pixel (if the text is not buffered).