how do I find out which 0845 numbers are chargeable?

[Christopher]

If you have a BT call package you do not pay for 0845 or 087 numbers (but you do to 0844)

You actually need to check the rules of your telephone service provider to find out what you will be charged for such calls - because it does depend on who you are with, and what sort of package you have. They all cost a lot more from mobiles.

You cannot blame the users of the numbers - they cannot tell you what it will cost, because there is no standard charge from all phone services. How much you will pay is up to your phone service provider, not the company you are ringing.