Whenever a little shrimp tucks itself into the plants, I get curious, and spotting a pregnant ghost shrimp is one of the best tiny victories in my tank hobby. The most obvious sign is the cluster of eggs under the abdomen — females carry them attached to their swimmerets (pleopods). Those eggs look like a little grape-bunch: usually greenish, yellowish or brownish depending on development and the shrimp's diet. If you can see a compact mass under the curved tail, that’s a berried shrimp.
Another clue comes earlier: a 'saddle' behind the head where the ovaries sit. It shows up as a translucent patch of color and you can sometimes see developing eggs there before they’re fertilized and moved under the pleopods. Behaviorally, berried shrimp will fan their eggs with their pleopods to keep them oxygenated, so you might notice more tail movement and quieter hiding spots. Eggs darken and show tiny black eye-spots as embryos form; if eggs turn opaque white or fuzzy they may have died or developed fungus. I always slow down water changes a touch and hide a moss mat when I see eggs — nothing thrills me more than watching those tiny dots turn into tiny shrimp.