Minimal example of effect

my-app.ts

/* eslint-disable max-classes-per-file */
/* eslint-disable @angular-eslint/component-selector */

import {
  ChangeDetectionStrategy,
  Component,
  effect,
  inject,
  Injectable,
  OnInit,
  signal,
  WritableSignal,
} from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class ServiceOne {
  public signalOne$ = signal(1);
}

@Injectable({
  providedIn: 'root',
})
export class ServiceTwo {
  public signalTwo$ = signal(2);
  private readonly serviceOne = inject(ServiceOne);

  public updateSignalTwo(): void {
    this.signalTwo$.update((value) => {
      return value + this.serviceOne.signalOne$();
    });
  }
}

@Component({
  selector: 'my-component',
  changeDetection: ChangeDetectionStrategy.OnPush,
  template: '',
  standalone: true,
})
export class MyComponent implements OnInit {
  public signalTwo$!: WritableSignal<number>;
  private readonly serviceTwo = inject(ServiceTwo);

  public constructor() {
    effect(() => {
      // / I want to spy on `updateSignalOne` and make sure it gets called when I update someSignalTwo$, which depends on someSignalOne$, but doesn't come from a service that is being injected into this component!
      this.serviceTwo.updateSignalTwo();
    });
  }

  public ngOnInit(): void {
    this.signalTwo$ = this.serviceTwo.signalTwo$;
  }
}

Working test using ng-mocks

describe('MyComponent', () => {
  const mockSignalOne$ = signal(1);
  const mockSignalTwo$ = signal(2);

  const updateSignalTwoSpy = jest.fn().mockImplementation(() => {
    mockSignalTwo$.update((value) => {
      return value + mockSignalOne$();
    });
  });

  beforeEach(async () => {
    return MockBuilder(MyComponent).mock(ServiceTwo, {
      updateSignalTwo: updateSignalTwoSpy,
      signalTwo$: mockSignalTwo$,
    });
  });

  it('should update signalTwo when effect runs', () => {
    const fixture = MockRender(MyComponent);

    mockSignalOne$.update(() => 2);
    fixture.detectChanges();

    expect(updateSignalTwoSpy).toHaveBeenCalledTimes(2);
  });
});