Drop a cartridge into the chamber to check how flush it is with the breech; it should be only a few thousandths below flush. If you do any filing on the breech it would affect that. Set a small square around the breech swell and see how it looks across that breech; even or lopsided, etc. Hold it out and look at it; the human eye can detect more than we often give it credit for. Set it up in the frame and close it up as far as t will go. Does it look straight and square and even, just doesnt close up far enough?
Do not remove any metal unless you know it is without a doubt the thing to do.
If it passes the above inspections you are ready to proceed to step 2.