Writing tests for epics with state$ and using marble diagrams. More information on marble testing can be found at https://github.com/ReactiveX/rxjs/blob/master/doc/marble-testing.md

epicMarbleTest.ts

import { Action } from "redux";
import { ActionsObservable, Epic, StateObservable } from "redux-observable";
import { TestScheduler } from "rxjs/testing";

const assertDeepEquals = (actual: any, expected: any) => {
    expect(actual).toEqual(expected);
};

export const marbleTest = <T extends Action, O extends T = T, S = void, D = any>({
    epic,
    actions,
    states = "",
    expected,
    values,
    dependencies,
}: {
    epic: Epic<T, O, S, D>;
    actions: string;
    states?: string;
    expected: string;
    values: { [marble: string]: T | O | S };
    dependencies?: D;
}) => {
    const testScheduler = new TestScheduler(assertDeepEquals);

    testScheduler.run(({ hot, expectObservable }) => {
        const state$ = new StateObservable<S>(hot<S>(states, <{ [marble: string]: S }>values), <S>values.s);
        const action$ = new ActionsObservable<T>(hot<T>(actions, <{ [marble: string]: T }>values));
        const output$ = epic(action$, state$, <D>dependencies);

        expectObservable(output$).toBe(expected, values);
    });
};

pingEpic.test.ts

import { marbleTest } from "./epicMarbleTest";
import { rootReducer, pingEpic, ping, pong, setMessage } from "./reducer";

test("ping epic", () => {
  const s = rootReducer(undefined, { type: "" });
  const t = rootReducer(undefined, setMessage("world"));

  marbleTest({
    epic: pingEpic,
    actions: "a -- a",
    states: " -- t",
    expected: "5s 1 -- 2",
    values: {
      a: ping(),
      s, // s is used as the initial state, doesn't need to go in the marble
      t,
      1: pong(s.message),
      2: pong(t.message)
    }
  });
});

reducer.ts

import { combineReducers, Reducer } from "redux";
import { Epic, ofType } from "redux-observable";
import { mergeMap, take, map, delay } from "rxjs/operators";

export const PING: "PING" = "PING";
export const PONG: "PONG" = "PONG";
export const SET_MESSAGE: "SET_MESSAGE" = "SET_MESSAGE";

export const ping = () => ({
  type: PING
});

export const pong = (message: string) => ({
  type: PONG,
  payload: message
});

export const setMessage = (message: string) => ({
  type: SET_MESSAGE,
  payload: message
});

export type AllActions = ReturnType<typeof ping | typeof pong | typeof setMessage>;

export type MyReducer<S> = Reducer<S, AllActions>;

export const messageReducer: MyReducer<string> = (state = "Hello", action) => {
  if (action.type === SET_MESSAGE) {
    return action.payload;
  }

  return state;
};

export const rootReducer = combineReducers({
  message: messageReducer
});

export type MyState = ReturnType<typeof rootReducer>;
export type MyEpic<O extends AllActions> = Epic<AllActions, O, MyState>;

export const pingEpic: MyEpic<ReturnType<typeof pong>> = (action$, state$) =>
  action$.pipe(
    ofType(PING),
    mergeMap(() =>
      state$.pipe(
        take(1),
        map(state => state.message),
        delay(5000),
        map(pong)
      )
    )
  );

What versions of the libraries are you using? This doesn't type check for me with

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Edit: fixed this by adding

interface Values<T> {
  [marble: string]: T;
}

then change the following lines

    const state$ = new StateObservable(hot<S>(states, values as Values<S>), values.s);
    const action$ = new ActionsObservable(hot<T>(actions, values as Values<T>));

This was for typescript 2.8 I think. In my project I added a typecast like you did, updated the example.