You can save transaction hash before sending it and then query Toncenter API method getTransactionByInMessageHash to check if transaction with such hash was confirmed or not

Example:

// Sleep function:
const sleep = ms => new Promise(r => setTimeout(r, ms))

// `msg` is a Cell containing your external message
// Convert message Cell to BOC String
const boc = await msg.toBoc(false)

// Calculate it's hash
const hash = tonweb.utils.bytesToBase64(await msg.hash())

// Send message and run a loop until transaction with that hash confirms
await tonweb.sendBoc(boc)
var txs = []
while (txs.length == 0) {
    await sleep(1200) // some delay between API calls
    const resp = await fetch('https://toncenter.com/api/index/getTransactionByInMessageHash?&include_msg_body=false&msg_hash=' + encodeURIComponent(hash))
    txs = await resp.json()
}

console.log('Done!')