You can't switch water supplier like you can gas and electric. So it's simply a case of finding out which water company supplies the area in which you live.
You should be able to find out either from the previous tenants or the landlord if you've just moved in.
Alternatively, if you live in England or Wales you can check the Consumer Council for Water's website to find out which water company supplies your home. If you live in a border area then you should contact the water company directly for confirmation (a full list of contact details is available on the Ofwat website).