Here's why:
* Flat head: Slugs have flat, broad heads, unlike snails, which have more rounded ones.
* Slimy trail streak down back: Slugs secrete a slime trail for movement, which can leave a visible streak on their backs.
However, a snail can also leave a slime trail behind! It's not unique to slugs.
Let me know if you have any other details about the creature. A photo would help!