well, you know him very well. you can probably predict, based on his history, whether he's in recovery mode, or relapse mode. i think it's very common for the resentments to be the catalyst for the relapse. you probably know that the relapse begins before the first drink is taken.
unless he's gotten really good at pretending, showing you the behavior that he knows you will use as a barometer of if he's sober or not. but most don't, their thinking gets messed up and they (mine anyway) are just poopy or angry all the time.
it's been really hard for me to just wait and see. it's angst-provoking, but if he's doing well, eventually you'll really know that. and if he's not, you'll know that too.