1) The Face: Cougars- have a round face, with a wide nose, but short muzzle. They have a dark streak on either side of their muzzle, and white around the mouth.
Bobcats- have a square face, with fringe of fir on the sides of the face. The face has more spots and lines than a cougar.
2) Ears: Cougars- have rounded ears with black behind the ears.
Bobcats- have ears which look pointed because of black ear tufts. The backside is dark, with a white spot.
3) Tail: Cougars- long tail that is almost the length of the body about 30 inches, hanging low to the ground with a curve and black tip.
Bobcats- short stubby tail that is white and black striped. The "Bob"cat is named for the short or "bobbed" tail.
4) Coloration: Cougars- Tawny (light brown) to a darker brown or gray. They have white underbelly. Juveniles will have spots, which will disappear in 1-2 years.
Bobcats- Gray and white, with some brown tints. They have prominent black stripes and spots. Stripes on their legs, and spots on their backs.
5) Size: Cougars- are large cats which males can grow 115-200 lbs, and females 75-150 lbs. Their adult size is 60-76 cm tall, and 5-9 feet from nose to tip of tail.
Bobcats- adult males can be 70–120 cm long with a tail size of 4 inches. Length of a bobcat is between 30-35 inches. Females weigh about 20 lbs.
Juvenile mountain lions can look similar to adult bobcats, except without stripes, short tail, and the facial mane of a bobcat.
References: Cougar photos: TRACE projects, Camera Trap Codger
Bobcat photo: flicker creative commons