Knowing when your sauce or other liquid has reduced enough can be a tricky thing to gauge. With a jury stick, you can check in a matter of seconds. Photo by goblinbox_(queen_of_ad_hoc_bento).
Reduction, the process of heating a liquid to thicken or intensify its flavour, is something you usually have to play by ear. To make it a little bit less of a guessing game, Emma Christensen at The Kitchn recommends you use a jury stick:
Make a jury stick by wrapping a rubber band around a chopstick or skewer and inserting it in the liquid when you first start reducing. Roll the rubber band down the chopstick until it touches the surface of the liquid -- this is your starting point. As the liquid reduces, occasionally insert this jury stick into the liquid and use it to tell how far the liquid has reduced.
This method is best for when you need to reduce a large amount of liquid. For a smaller amount, you can just tilt the pan to visually gauge how much water you've cooked out. Lastly, if you're just trying to intensify a sauce's flavour, you can use taste as your guide. For more clever reduction techniques, check out the link below.
3 Ways to Know When Your Sauce Has Reduced [The Kitchn]