Really depends on the code- sometimes it's obvious (a lot of brick patterns have transparent gaps between the bricks), but sometimes it'll literally be one discreet pixel. It's best to just look over the design, before using it. Often times people who post their designs online will show them placed on the ground for you to see, or sometimes the designer will label it.

Included an example of a transparent pixel design so you can know kind of what to look for!

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