I have not had much luck searching for how to tell a reblue on the board.
Since you are having trouble, and may be on a time crunch, Ill see what I can do to help, if just a bit. The markings should be crisp. If you want to know what I mean by crisp, look for posts on the forum of NIB, or new in box examples that are posted all the time on this forum. If the edges of the markings are soft, its a danger sign.
For me one of the most obvious ways is to look at how the sideplate matches up with the frame. If that mating of sideplate and frame looks like a "valley" then its almost certainly a refinish in my book. Its from when the refinisher takes the side-plate off and rounds the edges in the polishing process. If you are unsure of what a side-plate is, its the flat piece that is on the same side of the frame that the cylinder latch is on. It keeps all the parts from falling out! If that still doesn't make sense, go to Numrich website and look at the schematics and you will see what I mean by side-plate. Another thing that some guys are good at seeing are dished screw holes. If it looks like the screw is sinking into the metal, take caution.
Like others said, the Rampant Colt on the side-plate side is a good starting point. If should be crisp.
Sometimes the color is off. Colt handguns do not all have the same finish color. So you will need to learn what the bluing looked like from that period to compare properly. I am no expert, but if I see an 80 year old Colt, and it looks like someone painted it with high gloss black nail polish, Im going to think its a refinish.
Im not an expert, just trying to pass on my own observations from my limited experience.
I would not pay 600 for a refinished DS. I would pay 600 for an original condition one in high condition if I was wanting a DS. For a high condition, ALL-ORIGINAL, I do not consider 600 to be too far out of line. In my neck of the woods, you dont see many Detective Specials.