I'll assume you mean the visual center of the assigned sprite.
If so, that depends on various factors. These are the instance's position and its assigned sprite's width, height and origin. You can incorporate these into a formula to find the visual center of the instance in regards to its drawn sprite.
Start at its position, add half of its sprite's width and height, respectively, then subtract the origin.
x = inst.x + inst.sprite_width/2;
y = inst.y + inst.sprite_height/2;
// inst = the instance that you are colliding
// you can use "other", depends on how you are performing the collision. It also depends on other factors such as sprite alignment.