after looking at a bunch of examples heres the rule of thumb i came up with, like all rules of thumb it is not exact and one would not have to work real hard to find exceptions.
but if you take the ratio of the dielectric constants, putting the biggest one in the numerator.
if the number is less then 4 they will be miscible or nearly miscible.
if it is between 4 and 7 they will be somewhat soluable.
if it is greater then 7 they will be insoluable.
given that the dielectric for bromoethane is about 10, and that for dmso and dmf is 47 and 38 respectivly, we'd expect EtBr to have a fairly good solubilty in either one, with dmf probobly being the better one.