Hi @anon86102203 - very common struggle. Great question.
Even though Asana doesn’t offer this feature natively, there’s a fantastic tool called Asana Custom Field Explorer, built by @Bastien_Siebman. He has a very helpful free version of many of his tools, and several advanced/premium tools too. You can visit the link above and use it for free immediately, or you can read more about it here: List all custom fields of an organization - #20 by Bastien_Siebman
He’s a great developer, if you need anything custom built.