You can check default fee token order on-chain, go to Polkadot Webapp - Select the chain (Acala or Karura)
Navigate to Developer - Chain state > Constants >transactionPayment defaultFeeSwapPathList
On Acala, the order might be ACA > aUSD > LCDOT > DOT.
If a user has no ACA balance, then aUSD will automatically be used as fee token.
Users can set their next default fee token to other tokens by executing the following transaction:
transactionPayment.setAlternativeFeeSwapPath(fee_swap_path)