One thing I noted for sure is that Manga usually is not drawn in the same style they use for X-Men, Batman, Superman (DC Comics or Marvel).
They tend to be drawn slimmer & less muscular. When they are drawn muscularly, the muscles tends to have more packs (not sure what's that called) than that in American comics. A simple comparison is between Goku of Dragon Ball and Superman. Goku has more packs than Superman.
Also, Manga character hair tends to be spikey, while American comics are not. For example Tony Stark of Iron Man and Natsu Dragneel of Fairy Tail.
Manga tends to draw the characters with big, round eyes. This is especially true in girls manga (shoujo manga).
Well, American manga don't draw jaw-dripping expressions like manga do, or panic character having >.< eyes.
Another this is that effects in American comics are usually drawn inside a spikey bubble, while in manga, they are usually just written in big hiragana or katakana letter.