You'd need known genuine high-res images to start, and ultimately need both in-hand. There are a lot of steps and there are people who specialize in authenticating these.
Sometimes paper can be cleaned and the ink removed, and then reprinted, even newsprint. From the look of it, that could be the case here. It's also strange that the ink of the flyer is the same shade of black as the ink of the pen, unless that writing was supposed to be part of the original poster.