Nature poems tend to focus entirely on nature-on how it looks, smells, feels, makes people feel, how it inspires fear, love, dread, awe and amazement in people. In the poem "To Autumn" by John Keats, the first clue that it is a nature poem is the title-autumn is a part of nature, and the poem is addressed to autumn itself, so, unless he is writing to a woman named Autumn, or put in a very misleading title, we can hazard a guess that it is a nature poem. But as we look more closely at the text, we can see that Keats goes into great detail to explain autumn and many other aspects of nature.
Here are some parts of nature that he includes in his poem: "vines", "apples", "moss", "kernels", "flowers", "bees", "sun".... and that is just in the first section itself. Nature poems tend to have a lot of nature in them, and in the first stanza alone, there are mentionings of many, many aspects of nature.
In this poem, Keats also personifies autumn, giving in human-like traits. He says that autumn conspires, sits, sleeps, spares, keeps, patiently looks, thinks, and watches. Nature poems often personify nature in some way, making aspects of nature seem like they are alive, sentient, thinking forces that act and behave in ways like we do. This makes them seem more alive and interesting, and helps us to relate to them. So that is another way that Keats makes this a nature poem. I hope that those thoughts help a bit; good luck!