how can I find 'max' on my night storage heater?

The storage heater in my rented house appears to work well, but the two control dials aren't calibrated - so I can't tell where 'max' is. The diaks are like buttons with a groove down the middle. On the casing around each dial is a circle with a mark at 6 o'clock, but I don't know if this signifies 'max' or 'average' - it's confusing. I'd appreciate hearing from anybody else who has this kind of storage heater.