A block explorer is a website that lets you view transactions and addresses on a blockchain, such as Etherscan for Ethereum.
After sending, copy the transaction hash, also called the txid, from your wallet.
Use the explorer for the correct network, such as Etherscan for Ethereum or a Bitcoin block explorer for BTC.
Paste the transaction hash into the explorer search box and view the result.
The explorer shows the status, confirmations, amount, fee and the addresses involved.
Checking the explorer lets you independently verify a transaction went through, even if your wallet shows a different status.