This is where it comes to kind of a "feel" for it. A few tips are the filler should feather off very gradually. If you see sharp or crisp edges on the filler to metal, you probably have sanded too much. In other words, if the plastic filler turns sort of transparent around the edges it is likely feathered off nice. If it is an abrupt end then the shiny metal starts, you probably have sanded the filler down below the metal height.
Take a smooth piece of metal with no dents. Apply a thin coat of plastic filler to it. Now sand it down, feeling it along the way till it is gone. This will teach you to recognize how to feel the filler. If you know some is till on that straight metal because you can see it, but you can't feel it, then you need to get more serious about feeling it.
Don't feel with the tips or your fingers or something. It is a common mistake for "newbies" to stick their hand straight out and rub the panel side to side in front of them. You can't feel a golf ball glued to the surface like that! You need to stand along side of the panel and make love to it. Rub the panel with a very light feel, from one end to the other of the repaired area. You need to start off metal, rub across the filler onto the other side on metal. Get a feeling of the flow of the panel. Go to another point that is identical like the other fender and feel the exact same place. See what it should feel like undamaged.
