I am using a Free plan for IBM Watson Natural Language Understanding. I'd like to check the remaining credits of my plan for the given resource with a single API call.
I've seen something like Get credit pools in the IBM Cloud docs, but I am not able to determine my billing_unit_id which is required to make the request. Is this even doable for a Free plan? If so, could you help me with where to find a billing_unit_id or redirecting me to another API Endpoint (that hopefully has a cURL example) so I could check remaining credits of my NLU instance?
What you referred to is part of the billing and sage API for so-called Enterprises. An IBM Cloud enterprise is comparable to a meta account which allows to manage a hierarchy of accounts and account groups. It is not available for free and regular accounts and is unrelated to NLU usage.
What you are looking for is a way to see your usage for the IBM Watson Natural Language Understanding service. The usage in NLU items is returned in each API response.
For total usage reporting, you should check out the Billing and usage section in your account. There you can drill down. If you then have identified details, look into the usage reporting API or the CLI commands.