Can I read using an analog pin on an arduino Mega whether or not there is a high (5v) voltage on a wire from another board's digital pin?
Yes of course you can. Better to use digitalRead() than analogRead() however.

You just need to tie the grounds of the 2 Arduinos together, and set up the pin on the Arduino you want to sense the input as INPUT or INPUT_PULLUP. Then do a digitalRead on the pin. It will return LOW if the other Arduino is setting the pin LOW, and HIGH if the other Arduino is setting it HIGH.

And, as Juraj points out in their comment, CMOS devices like Arduinos respond to voltage, not current. Their inputs are capacitive. They react to a voltage potential, not current. (When you initially change a CMOS input from one state to another a small amount of current flows in to change the state of the capacitor and after that the only current that flows is a very tiny amount of leakage current through the capacitor on the input.)