I built a private testnet with reference to https://github.com/moneroexamples/private-testnet. I want to check the public tx key of the issued tx, but I don't know how to check it. I cannot use https://xmrchain.net/tx/[tx_id] because I use a private testnet. How can I check the public tx key?
The public tx key is contained in the extra field. You can use monero-utils-deserialize to decode it from the transaction's hexadecimal representation, which you can get from the daemon, like this:
print_tx TXID +hex
(replace TXID with your transaction's txid)
Then, in a shell:
monero-utils-deserialize --input DATA
(replace data with the output from the previous monerod command)
If the transaction contains a tx public key (and the vast majority do), it will be printed out.
Note that monero-utils-deserialize is only built for debug builds.