The juiciest lemons and limes are always going to be the ones that give a little when you squeeze them. These softer citrus fruits will have less pith, and therefore more juice, than their less-pliant companions. They're also much easier to juice and use in cooking or baking.
Don't worry - we're not talking about mauling the lemons or causing their skin to break. Just picking one up and gently pressing it with your thumb is all it takes to tell if the fruit is soft or hard.
As a side note, while this technique works well for picking the best lemons and limes, it's less helpful when choosing larger citrus fruits like grapefruits, oranges, and pomelos. Those citrus fruits have naturally thicker piths that make it harder to tell the fruits quality just by squeezing.
(Image: Emma Christensen)