How Do You Tell An Adjective From An Adverb?

Sometimes they can be the same - "he drives fast/he's a fast driver" - but this is quite rare.
The easiest way is to look at what they're describing. An adjective describes a noun (name: Jim, the table...) or pronoun (I, he...) So in sentences like "The table is green" or "She feels happy" the adjective is easy to identify.
Adverbs usually describe or modify verbs ("He walks quickly") and the simplest ones end in "ly". Even if they don't, however, you can identify them by what they do in the sentence; adverbs are about HOW something is , while adjectives are about WHAT it is. In "He always leaves at 10," "That's very good" and "We've just finished", "always", "very" and "just" are adverbs; they give information about manner, time, place, amount etc.
Compare these two sentences:
-"I want to make a direct call"
-"I want to call direct"
The first "direct" (adjective) defines a type of call; the second (adverb) describes the manner in which I want to do it.