The term "slug" refers only to the body shape adopted by a shell-less (or nearly so) mollusc. Detailed study of their anatomy reveals that they have evolved several times from different ancestors. They are not a natural group of closely related animals. Some slugs are more closely related to certain snails than they are to each other.
A saddle-like mantle extends over the anterior part of the back of the slug, and this bears a "breathing hole", or pneumostome. If you cannot see a mantle or a pneumostome on your specimen, have a look at the hind end and make sure it does not carry a tiny plate-like shell. If it does, you may have Testacella, so click here.
Some slugs have been accidentally introduced abroard and have become pest species — read Slugs: A Guide to the Invasive and Native Fauna of California which includes keys, details and photographs of many species introduced from Europe, some of which are included in the following pages.
If you CAN see a pneumostome on the mantle, click on the image below.
A typical slug