I think dropping a combo and then preceding to not make any movement at all after being very high pressure in a match is a very good indicator of a dead battery
I think dropping the most basic bread and butter combos of all time is a hint enough. I think I know who you mean, but he's always going against this guy who really has a stick up his arse and only attacks when his opponent cant fight back :/ Even when our friend tells him that his controller will randomly cut out in a match, this guy will still attack when said controller fucks up :/ Some people, eh?
When I see the opponent is suddenly not moving I'm gonna wait for a while. If it takes too long I will just smack him all the way into the lobby but still. Some of these derps think they're cool when they win rounds on people who don't move.