When a sauce is spread out over the bottom of a shallow pan, it's hard to really judge if I have a whole cup or just a few tablespoons. Tilting the pan pools all the liquid to one area and makes it much easier to estimate the amount. If I remember to tilt the pan to check the amount when I start reducing a sauce, this also makes it easy to tell when the sauce has reduced by half.
The more often I do this, the more accurate I find that I become. Do you use this trick, too?
Related: Technique: Making a Sauce Reduction
(Image: Emma Christensen)