Generally with off-peak immersion heating you have effectively two immersion heaters. One is fed from the off-peak supply and one with the 'normal' supply. If you have two obvious immersion heaters then the lower one of the two is the off-peak and the upper one the 'normal' and you should be able to work out which one connects to which switch.
However if you have a dual element immersion heater then its not obvious. Dont know what kind of metering arrangement you have but if you reduce your usage by switching as much as you can off in the house you should be able to see some response on one or other of the meters when you turn one of the switches on.