You could read ":" aloud by simply pausing: "For all a in A, a in B." Or you could read it as "it is true that."
Here's another way.
if and only if
Edit: upon reflection, what I wrote above:
isn't precise enough, because is unspecified, thus the expression is not a sentence/proposition in the language (it cannot be assigned a truth value); we would still need the symbol, like I did in this post.