How do I know if the water tank in the loft contains asbestos?

Best thing to do is to get someone that knows about asbestos. But it will be like grey colour and alot of old house's will have them normally plastic now

Arrange for an Asbestos survey of property if of concern or contact licensed asbestos removal contractor to assess. If your property was built before the year 2000 there is a chance elements of the build have used Asbestos associated materials. All the best Jengis