How do imagine this would working without using guides or grids? The way I normally do it is:
1. Drag out a guide to point A
2. Drag out a guide to point B
3. Hold shift and put the mouse cursoe between the two -- it shows you the distance
You could also use the measuring tool (click and hold the rectangle tool in the tool pallette.)
Or even just draw a rectangle and look at the width in the property panel. That's a fast and easy method I use sometimes.