You can do this in a couple of ways:
Look at the planning history of your building or land
It may be hard to identify the permitted use of a building or piece of land because its history might be complicated; for example, planning permissions may have been granted in the past, but they may not have been implemented.
A search of historic planning decisions will identify what planning permissions exist, and this may identify what the existing Use Class is. If you are in doubt, apply for a Lawful Development Certificate.
Apply for a Lawful Development Certificate
If you need confirmation that a building or piece of land has a certain Use Class, you can apply for a Lawful Development Certificate.