That is a simple as:
List<B> blist = A.getYourAssocNameFromAtoB();
If you are using Eclipse, Ctrl+ Space is your friend. If you are not using Eclipse, you should.
update
The reference is not stored on the A side, so its not directlty accessible there. You should use the core.retrieveXPathQuery api.
For example:
List<IMendixObject> Bs = Core.retrieveXPathQuery(this.getContext(), String.format("//%s[%s= %s]", B.getType(), B.MemberNames.A_B, a.getMendixObject().getId().getGuid());
Optionally you can loop afterwards to instantiatie proxies for B.