The Ethereum protocol is defined such that you can trust that the sender is really set to the owner of the account (in Solidity, that means you can trust
msg.sender is already validated). Below are some sections of the yellow paper which define this protocol.
Specifically, a transaction includes several parameters (
s) that allow everyone to validate the signature.
Transactions are only considered valid if the signature is valid. (see item 2)
Blocks that include invalid transactions will be rejected as invalid by protocol-respecting miners and peers. (see item 2)
Miners that attempt to include a transaction with an invalid signature anyway (violating the protocol) will waste their resources: future miners will not build on top of the invalid block, and the malicious miner will never get to spend their block reward.