Here's the trick that I was taught years ago:
Once you've reached the point where you're sure the seam is gone, hold the piece up so you're looking down it at about a 30%-45% angle and catching good strong reflected light. You should see a uniform surface no matter how you turn or roll it around. If you see spots that are shinier, or rougher than the rest, or still see an actual seam line, go back and sand/polish some more. Once the whole kit, or subassembly, can pass this test (totally even, shiney surface with only things like panel lines interrupting the scenery), it's time to shoot your primer coat. Once the primer is filly cured, do the reflection test again. If you see anything that doesn't belong, sand again, then reprime. Continue until all the flaws have been found and fixed.
Sounds labor intensive, and the first few times you do it it's kinda a pain in the rear. But discovering a flaw after a color coat goes down costs more time, and aggrevation, to fix than if it were taken care of in the first place.