There is a chain ID for each chain that is unique. You can retrieve this with the get info API/cleos call.
You also enter this if you're say loading a new network into scatter.
cleos --url <API ENDPOINT HERE> get info
http://127.0.0.1:8888/v1/chain/get_info
This is the main net:
{
"server_version": "2fc5d1cb",
"chain_id": "aca376f206b8fc25a6ed44dbdc66547c36c6c33e3a119ffbeaef943642f0e906",
"head_block_num": 35934160,
"last_irreversible_block_num": 35933831,
"last_irreversible_block_id": "02244e87d51eefbce234da2f8238e8df20aff331bc9c66081e3d7b702bb7f178",
"head_block_id": "02244fd02601f1067306775c02c4ec4ae8fb796ad375af66cdaa86cc6dcc048d",
"head_block_time": "2019-01-06T02:06:51.500",
"head_block_producer": "cochainworld",
"virtual_block_cpu_limit": 34468896,
"virtual_block_net_limit": 1048576000,
"block_cpu_limit": 176723,
"block_net_limit": 1045864,
"server_version_string": "mainnet-1.5.0-dirty"
}