The easiest way to is look at the player's profile to see their score. If yours is a noticeably higher you will lose points. If your is noticeably lower you will gain points. If both scores are similar, the elo score will be the same.
Forgot to mention in the old version of the rules there was a tool under the elo section that calculated how your score would be affected for a win, loss or draw.
I'm not sure if the link will work but here it is.
Then go under the elo mode link.
It would be a good feature if it showed your opponent's ELO sometime during the battle. Like in the startup animation (someone vs someone), or by hovering over your opponent's name during the fight. This could change to suit each game mode, so you could see where your opponent sits in the DT, or how far they are on their survivor run.
I'm no programmer though, so I don't know how difficult something like that would be, even if it does seem quite insignificant. Just my thoughts. Happy UR'ing all!
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