It's a good question - even marine scientists often make errors. Manta rays are born at approximately 1.4 to 1.8 m 'wingspan' so they're of a similar size to most mobula species.
The other respondents have given you excellent information: manta rays of either species have much more pronounced cephalic fins than mobula rays, and mouth position is a definitive character if you get a good view.
An easy way to tell them apart in the field is also to just look for spots on the underside. Manta rays will always have spots/blotches/defined markings between the gills or on the stomach, whereas mobula rays don't. Manta rays, excepting the black (melanistic) colour form, also have fairly well-defined white/grey shoulder markings which are rarely present in mobula. Also, manta rays (while they may feed in close proximity to one another) don't school to the same extent as mobula rays, so if you see a bunch of them swimming together then that is also a useful pointer.
Hope that helps!