How can I make the Arduino sense two units 'collide' in GLCD?

I am making something a bit like 'Space Invaders' game with Arduino Nano and GLCD(AX07023).
However, I can't find out how can I make the Arduino sense that the enemies were hit by the bullet.

I found an example code using 16x2 character LCD(code attached), but I don't know how should I apply the method to GLCD. Or is that method even applicable to GLCDs?

Helicopter.ino (8.7 KB)