When your client connects to TWS/IBGW it receives callback with a list of the accounts the client application can trade through that connection. That list, obviously, depends on the active login TWS/IBGW has with IBKR at the time of client connection. So you can tell your client application which accounts are paper accounts and which ones are live accounts.
I could swear I have seen an IBKR Knowledge Base article in the past where they described how account numbers for various account types are structured but could not find it right now (maybe someone has a link handy?) If I remember correctly, paper accounts can be identified by their account number as well.
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