You can use assertThat.
We actually can now test for toast messages using robolectric. The example below is how our team is doing this for now:
@Test
public void ccButtonDisplaysToast() throws NullPointerException {
Button ccRedButton = (Button) findViewById(R.id.cc_red);
cc_red.performClick(); --> this calls the actual onClickListener implementation which has the toast.
ShadowLooper.idleMainLooper(YOUR_TIME_HERE); --> This may help you.
assertThat(ShadowToast.getTextOfLatestToast().toString(), equalTo("TEST_YOUR_TEXT_HERE"));
}
Hope this helps