How do you know when your pot of boiling, liquid jelly has reached the point where it will jell once it has cooled? Get it right, and you've got delicious preserves for your family or beautiful homemade gifts. Get it wrong, and you'll end up with either syrup or candy.
In theory you can use a candy thermometer. When the jelly's temperature reaches 220F (at sea level), it should be done. In practice, I've found this to be the least reliable way to be sure that my jelly is ready.
Fortunately, there are other methods for testing jelly. Descriptions of how to do a jelly "sheet test," "spoon test," or "wrinkle test" can seem mysterious if you don't know exactly what you're looking for. Let's demystify those methods so that you can make jelly with confidence.