To query the TXT record for DMARC, you can use:
dig TXT _dmarc.example.org
To query for a particular record for DKIM, you would need to know the selector prefix. You will find it in the s value in an email’s DKIM-Signature.
For example:
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.org;
s=google; t=1615461277;
[…]
You would then query it as TXT:
dig TXT google._domainkey.example.org