How confident are you with minor electrical work like taking covers off to check wiring? If you aren't there's not much that can be said to help & you'd be better off getting an electrician in.
A start would be to find out the make & model of the heater & then it might be possible to find the wiring instructions to see if it's a single supply or dual supply type. To get very far you'd really need some way of measuring whether a cicuit is live - a voltage stick might be enough.