Is it possible to check if there is other networks within the same Preamble(HP)?

According to the manual, it states that:
If a packet is received, it checks the preamble(HP) then the Network (ID).

My question is:
If the preamble(HP) is matched, and the network ID isn’t, how can we confirm that is the case?

You can’t. There is not a broadcast value for the ID frame.