Account owners are responsible for keeping any documents that support a qualified or nonqualified withdrawal.
Be sure to keep bank statements or receipts to help with any potential question of a qualified expense. Be sure the beneficiary knows what is and is not qualified.
One way to consider documenting expenses is with a debit card from a low/no fee checking account. Diligently teach your student when this account can and cannot be used (qualified vs. non qualified expenses). Consider funding the account with a few months worth of needed money with a direct ACH disbursement from the 529 state website, then assure on each detailed monthly debit card statement that costs are on the approved (qualified) list. Keep the monthly statement copies for any later potential audit.
No statement or example on this site should be considered a specific recommendation for your personal situation. Investing and tax strategies each carry significant risks. Examples herein may not apply to your situation. Please consult your estate attorney, tax advisor, or financial advisor for personal advice.