Difference
If you all could tell me how to tell the difference between boxers and bulldogs, I'd sure appreciate it. The above instance with the pitbull was the one and only time I couldn't pick a pitbull out of a crowd. So, I think I'm okay in that regard. But I'm sure stumped on boxers vs bulldogs.
There's a BIG difference I think!
Bulldogs are short, VERY stocky, and have much flatter faces and more wrinkles than boxers. They often have naturally short or corkscrew tails. Ears are always left natural, never cropped.
American Kennel Club - Bulldog
Boxers are much taller and leaner than bulldogs, resembling greyhounds almost in their body style. They are considered a "flat-faced" breed but are not near as flat-faced as bulldogs. Naturally they have drop-ears, but they can be cropped too; most boxers that compete in conformation in the US have their ears cropped. The boxer naturally has a long tail (like a pit bull), but traditionally the tails are docked before a week old -- this is still a very common practice in the US. The vast majority of boxers you see in the US have docked tails.
American Kennel Club - Boxer
Picture of a white bulldog (6 months old) and a white boxer (2 years old).
Caney and Jackson | Flickr - Photo Sharing!
Now, the Bulldogs pictured above are also sometimes called English Bulldogs. But if you're talking about AMERICAN Bulldogs... that's a completely different breed.

In my opinion they look like a perfectly even mix between a Bulldog and a Boxer. They are slightly bigger than a Boxer, thicker than a boxer in body and appearance (not as "elegant") but not quite as thick as a Bulldog. They always have natural drop ears and a long, natural tail. I personally think Am. Bulldogs resemble Boxers more closely than they do Bulldogs, simply because of their height.
American Bulldog - Wikipedia, the free encyclopedia
I can see how it would be harder to tell Am. Bulldogs from Boxers though, if you're not familiar with the breeds. Body shape, head shape, natural or docked tail, markings and coat color can all be tips.