Listen to the breathing. Sleepers will typically take shallow, even, long breathes which can be difficult to fake, especially for an extended period of time.
Check the person's pulse. Some sleepers are sensitive to touch and may wake up. Sleepers generally have a slow, even pulse although this can be thrown off if they are dreaming. Pulse rate is difficult to fake and would be a good indication of whether the person is asleep.
Look at the eyelids to see if there is any rapid eye movement. Rapid eye movement is characterized by rapid movements of the eye beneath the eye lid, usually from side to side. The movements will be visible to an observer but you will need light to see them.
Listen for snoring. If you know the sleeping habits of the person you will know if they snore and how it sounds. If he is pretending to sleep and faking snore, he would have a hard duplicating the sound. However, you might be able to distinguish his fake snore from the real thing.