How to verify that signed transaction, is correctly signed?

Question

I want to decode and check if the given hex data for the transaction is valid and signed correctly.

The flowing is how I currently construct the transaction and convert it to hexadecimal format.

    let addProxy = api.tx.proxy.addProxy(PROXY, 'Any', 0);
    let remark = api.tx.system.remark("Test");

    let tx = api.tx.utility.batch([
        addProxy,
        remark,
    ]);
    let singedTx = await tx.signAsync(pair);
    let signedTxHex = singedTx.toHex()
    
    console.log("Signed TX", signedTxHex);

Answer 1

Do you need to do it in PolkadotJS? Otherwise you can use the tool Subkey: https://docs.substrate.io/reference/command-line-tools/subkey/ with the command:

subkey verify <your-signedTxHex> <pair.address>

Answer 2

As Jaco explained in this GitHub issue, you can't just verify the signature of a transaction with polkadot.js.

Alex's answer is a good alternative by using the tool Subkey.

However, if instead of transactions, you want to verify just messages, you can do that with the util-crypto package of polkadot.js. Here you have the documentation for it.