Checking account balance in Sidebar does not match Account Balance in Quicken or On-line Balance

You're right, there is and never has been a way to display the currency selection of a single account,

• not in the Edit Account Details dialog
• not in the Account List

You can only deduce an account's currency by looking at the currency symbol shown with the account's balance in the Account Sidebar or when looking at the account register's bottom line Current Balance or Ending Balance amounts.

The Account Sidebar Total is the sum of all account balances multiplied by the currency exchange rate in effect.

You are using the HB&R feature level. For feature levels higher than Starter Edition, the Tools Menu should contain the Currency List … unless you have never enabled "Multicurrency support" in Edit / Preferences / Calendar and currency.

In the Currency List, by default, the US currency entry should show "$" as the Currency Symbol and it should be marked as the "Home currency".
There should not be multiple currency entries using the same currency symbol. For example, use "$" for USD, "C$" for Canada.

With multicurrency support enabled, reports will show the selected currency in the report title.
If all transactions shown in the report are of the same currency, that currency name is shown, e.g., "in US Dollars". Otherwise "converted to US Dollars" will be shown and all transaction amounts are converted to US Dollar at the exchange rates listed in the Currency List. The report currency defaults to Home Currency or whichever currency you select in the report's customization dialog.

The long and the short of all of this is:
If the different account balance, as shown in the Account Sidebar, cannot be explained by the setting of Current Balance or Ending Balance, as asked for by @Quicken Kristina , then I can only assume an incorrect currency setting for the checking account. Something other than US$. And that's what I'm trying to find out.
So, if you would, please enable Multicurrency support and then review the Currency List's US Dollar entry: Needs to have an exchange rate of 1.00 and be marked as Home Currency (unless you'd rather use a different home currency).