Quantifying how good an alignment is?

I have an experiment where I am taking an image every 5 min of the same cell, but I want to monitor positional drift of my setup. I have an alignment program that scales the image based on an initial image.

I can plot the image with and without alignment, but I am looking for a method to quantify this. Is there a matlab function that can return a coefficient of alignment to check how good an alignment is?

[Rik]

If you have the displacement you could use the magnitude of that.