Long and diligent study. :) For the most part individual runs are not marked in any easy way to differentiate them. So you need to know what you are looking for. Even then it is easy to get fooled sometimes.
From time to time manufacturers change the standard hotstamps on discs but if you have a custom stamped disc you lose even that clue.
The best way of knowing how a disc will fly is to throw it.
If you are only concerned with the collector value it is best to acquire it from a very knowledgeable dealer and put a sticky note on it with the details of how the dealer described it. If the dealer will then sign the sticky note it is possible that one day the note will make the disc more valuable.
If your not getting a disc for collectability then your looking to recreate the disc you have or have just lost. Chances are that one is broke in and a new one, even fom the same run, is not going to be identical, close, but not identical.
" The best way of knowing how a disc will fly is to throw it. "