Well,
Entacmaea quadricolor has 4 hoofs and a long tail with a bushy end. Long shaggy fur and two black beady eyes placed under some pointy horns.
Heteractis magnifica on the other hand tends to have strong muscular back legs, sharp claws, a slightly tapered tail and very strong disposition to seek out heat registers to have a nap.
... Seriously, usually, there's no confusing the two, they're quite different. Yet sometimes you'll see these ones where they defy easy categorization. If it comes down to it,
H. magnifica has verrucae arranged in rows along the column.
E. quadricolor doesn't show verrucae along the column at all, although I think they might show some right at the top of the column.
http://www.nhm.ku.edu/inverts/ebooks/intro.html