How would you find out which standard a certain cryptocurrency is, that is whether the crypto is ERC-20, BEP-20 or something else?
Another convenient way to find out is to ask the administrators in their telegram group or social media pages that's if the tokens aren't listed on tracking sites like coingeckeo or coinmarketcap. Tokens today exists on multiple blockchain, for example, the stablecoin USDT is operating on the Ethereum blockchain (ERC-20), Binance smartchain (BEP-20, Binance chain (BEP2), Tron blockchain (TRC-20) etc
Projects also have wrapped version of their token on other blockchain especially those chains that are widely accepted, they do this most times to ease the listing of their tokens on decentralized exchanges and for easier utility. Do not that some projects do this without authorisation like for example Bitcoin has its wrapped version on this other blockchain but you're not advice to buy those as they're centralized and isn't the real Bitcoin.
Must a crypto have different contract addresses in different networks?
That's not a must and it depends on how the other blockchain is structured, for example I have seen project on Polygon and Binance smartchain sharing same contract address. Do note majority of this other blockchain are just copycats of the Ethereum blockchain so they have similarities when it comes to addresses.