Identifying mail marked as junk; marking and unmarking mail as junk
Many mail servers have their own spam filters and will add a special header to the message to indicate spam (some are yes/no, others will use a number system where higher numbers indicate the more likelihood the message is spam)

Select a message and Select View -> Message -> Raw Source from Mails menu to see all the headers and info. You may see something like this (highlighted):

 

Based on what you find, you may set up a rule based these headers. (in Mail -> Preferences -> Rules, you may need to add the header in mail (see img))



Repeat this for each Mail client.

As for Mail.app's custom heuristics... That's a bit trickier, but if you are consistent they should end up pretty close on each machine. (I tend to trust my server's filtering more anyway)