I know there is no "from" address when we send bitcoins. However, is there any way to prove when B receives btc, it was sent from a particular party? If A and C both claim they sent btc to B with the same amount and B only receives one payment, how do we identify the sender?
You can definitely see the addresses that the Bitcoins were sent from on the blockchain. The actual owner of the address(es) that sent the payment to B could then simply "sign a message" proving that they control the private key for that address and therefore "own" it.
It's the reason why a lot of airdrops and ICOs etc want signed messages when claiming coins/payments... It proves control/ownership of a given address.