Depending what you are trying to do, you can check the number of joins within 1 second. If the number is > something such as 5, you can assume it's a rejoin from a netsplit. Like I said, this all depends on what you're trying to do. If you just want to know for sure if it's a netsplit rejoin or not, this won't help as you can have 1 person rejoin from a netsplit. But if you just want to lock the channel if more than X people join in something like 5 seconds or something like that, you can use this idea to ignore it if all those people join in 1 second.
