How do I check which payment method is associated with my Apple Account?

This article tells you how you can view the payment method associated with a particular account but it won't tell you if it is associated with other accounts too: If you need to change or update your Apple Account payment method - Apple Support

It can be more helpful if you outline what the problem is prompting this question. If you are wondering how items are charged on an account:

From this support article --> How apps, content, and subscriptions from Apple are billed - Apple Support

"Which payment method is charged?

When you buy items or pay for subscriptions, Apple automatically attempts to charge your payment methods in this order:

1. Apple attempts to use your Apple Account balance to cover the full amount. This is at the top of the list always, but isn’t what is referred to as the “primary” payment method.

2. If you don't have Apple Account balance, you don't have enough to cover the full amount (balance will be used first, up to whatever it can cover), or an item cannot be paid for with Apple Account balance, Apple instead attempts to charge your primary payment method (typically a credit or debit card, but varies according to country). The primary payment method is at the top of the non-balance list in Settings on your iPhone. You can control which payment method is at the top of the list. [See: Add a payment method to your Apple Account - Apple Support ]

Don't confuse having to have a payment method present as an indicator it will be used. Apple may require a payment method be present even if it doesn't use it.

3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple may try again." See also sections B and H of Legal - Apple Media Services - Apple

If you are instead wondering about an unexpected charge on your account, the support article If you see 'apple.com/bill' on your billing statement - Apple Support and the related article Get help with charges from apple.com/bill - Apple Support explain how you can check your iTunes and App Stores purchase history, cancel subscriptions (remember that these need to be canceled at least 24 hours in advance), and track possible sources of unrecognized Apple charges you may see on a bill. These may be purchases by Family Sharing members, others sharing your card (such as a spouse or partner, or a second Apple Account you have; you may not see a receipt).

The article also provides a specific link for contacting Apple if you cannot trace the source of charges, e.g., if you subscribed to something directly with a third party, see: About purchases not billed through the App Store or Apple In-App Purchase - Apple Support instead. In that case you need to work directly with the vendor, not Apple. See also: Fabulous SAS - Unethical app, please dele… - Apple Community

Your payment method is not linked to any particular Apple account or individual purchase. Instead, Apple uses the payment methods listed under your Apple Account (formerly Apple ID) to process purchases and subscriptions.

If you have multiple payment methods registered, Apple generally attempts to charge them in the order in which they appear in your payment settings:

1. First payment method: Apple attempts to charge the payment method listed at the top of the list.

2. Insufficient balance or payment failure: If the first payment method does not have sufficient funds or the transaction cannot be processed, Apple may attempt to charge the next eligible payment method.

3. Subsequent payment methods: Apple continues down the list, where applicable, until the payment is successfully processed or no eligible payment method remains.

You can change the order of your payment methods. Simply go to your Apple Account's Payment & Shipping settings and rearrange the payment methods to suit your preference. The payment method you want Apple to try first should be placed at the top.

One important exception: If you have an Apple Account balance (funds added to your Apple Account), Apple will always attempt to use that balance before charging your other payment methods. You cannot change its priority by rearranging your payment methods.

In short, you are not assigning a particular payment method to a particular purchase. You are simply deciding the priority in which Apple attempts to charge your available payment methods.