With a little practice, recognizing sentences with imagery can become easier; chances are, you've read many sentences that contain imagery, and not even realized it. It is one of the most common writing devices used in many different styles of writing, in both fiction and nonfiction, because it really helps to add an additional layer to the piece. The key to recognizing sentences with imagery is to look for descriptive language that invokes one of the five senses. Visual imagery is the most common, which describes the way something looks, but other types of imagery might describe the way something smells, tastes, or sounds, for example.
Some types of sentences with imagery that may be more difficult to recognize are sentences that make use of metaphor or simile, which are other types of imagery. A metaphor is something that says one thing actually is another; for instance, "The sky is a painting." A simile makes a similar comparison, but uses the words "like" or "as" to make the comparison. In this example, the sentence might be written "The sky is like a painting." Metaphor and simile are two literary devices that take practice to use and recognize, but they add a lot of imagery to the writing, and often make it more enjoyable to read and easier to imagine.
Other types of imagery are more straightforward and easier to recognize. Visual imagery is the most common, and is simply used to describe something using visual language. Rather than saying "The coffee cup is sitting on the table," a writer might say "The sky-blue coffee cup filled with steaming coffee sat on the distressed wooden table." The sentences are essentially saying the same thing, but the second one allows the reader to picture the scene in a much more complete way, which helps to involve the reader in the story more.
The best writers will use many different types of sentences with imagery, and not just stick to visual imagery. Describing the way something smells, the way a certain sound affects a character, or the perfect way food tastes all serve the same purpose, to draw the reader into the story. Sentences with imagery allow the reader to become lost in the story, without needing to fill in the gaps by himself or herself. The best writers won't make the reader guess, but will provide enough imagery to give a complete picture that includes all the senses. Even though recognizing imagery will take some conscious effort, chances are once you get the hang of it, it will become very easy to see.