Study a photo of a slate roof. Print a copy of the photo, and take it with you as you walk around neighborhoods you like. The more time you study the characteristics of a slate roof, the easier it will be to recognize the roof when you see it.
Pick up a slate shingle. It is thin and fairly lightweight.
Take note of the color span of slate shingles. Slate shingles come in a wide variety of gray hues -- from light gray to deep gray to gray tinged with copper.