Centering a design in InDesign

I am using InDesign CS4 and am wondering how to center my design horizontally? I cannot tell where the center is because I am accounting for the gutter. If I am looking at a right side design page which colors lines (left and right) should I be using to find my center? Thanks!

Make an image box the full width of the page – there are small toggles at the corners and each side (at the centre points) simply drag a guide to the centre point then delete the object you created to find the centre points. Then align your image by lining up the toggle pints to the guide line.

I hope this helps
Best wishes

Carl Middleton