I just finished doing the same thing of trying to identify a number of N scale DCC decoders. And here's what I found out:
For Atlas, if you have the illustrated parts diagram, there should be different part numbers for either analog or DCC decoder board.
As peteski said, using the DCC decoder part number in Google, you might find threads, maybe from this forum, or others, that could narrow the search. I did find this link for Atlas:
http://www.atlasrr.com/pdf/Atlas_N-Scale_Decoder_Applications.pdfAnd I also found that early Atlas DCC equipped locomotives used Digitrax decoders that had the Manufacturer ID of 127 for Atlas and not 129 used by Digitrax. This could present problems during programming and where JMRI DecoderPro might also help in identification.
The other thing I discovered was that DCC decoders made by other manufacturers, in some cases, might revise the format of part number on the DCC decoder (DN163K1a, LE063XF, N12A0) using only a couple of numbers or letters of the original. Some could be self-evident, others may require some CSI investigative research.
Hope this helps...